If the player wants their old character to reappear as an NPC, then they have to accept that that character is now an NPC. That means you are playing that character—and you get to play them according to your perception of the character.

This can be fraught with potential for argument and disagreement about what that character would do; most DMs clear the reuse of former PCs with that player, and come to an agreement about things. But it’s an agreement, not the player getting to use their former PC as a fall guy. The old character is much higher level, much more important, much richer. They were an adventurer of some description. They can and will defend themselves. The new character can still attempt the mugging, of course, but it has to actually be attempted, something you actually play. Feel free to add protections to them as appropriate for the intervening time—wards, alarms, bodyguards, whatever. Presumably, this won’t go well for them; in fact, it seems likely that it going badly will be so obvious that the player will beg off from actually trying it.

Or if the old character really is that stupid, then someone else has already stolen all their stuff. After all, it has been a long time—either they have protected their stuff or it’s all long gone.

If the player doesn’t agree with that, then their old character doesn’t reappear in the campaign. Called away from Waterdeep on urgent business or whatever. It’s a shame, but it’s better than the alternative of arguments and strife about what the character would or would not do.

Apenas diga não

This is egregiously self-dealing meta-gaming.

Eu gosto esta resposta a lot, in spirit, and my guess is that it will work more often than not. But occasionally you will get someone who treats this like a logic puzzle: "Well but... what if Wizard Two is Wizard One's favorite niece? And he gives her the staff!"

Do not be afraid to just say, "No, that's not happening, that's not how this works, that's not how any of this works." Make it stick, and move on. How your player reacts to being told no, politely but firmly, will speak volumes about his character. And I don't mean the one he rolled up to play.

It sounds like he's testing you. What follows is an in-game option you have for showing him that you aren't messing around; that, when you're DM, each party member will have strengths and weaknesses, and choices have consequences.

Allow the heist, but make it a pain in the butt that results in a net loss to the player.

Tell the player, "Old Man McMageFace isn't your character anymore. He's now just another high-level NPC. If you insist, though, just this once I'll allow a high-level NPC to part with his gear. Normally, said NPC would murder you in your sleep within 24 hours, but I'll even decree that Old Man McMageFace has completely lost his marbles and doesn't care that his stuff is gone. Mask, God of Thieves, appears before your new character, saying that your old character's equipment can be stolen without retribution from said character. The only condition is that you, the player, don't get to utter a word of complaint if it makes the game less fun." If your player is gutsy enough to say "Okay"-- well, read on.

Valuable things gravitate toward owners who can hold onto them

Why do stronger monsters have better loot tables? It's not just about game balance; it also makes for a more believable story. Consider three possibilities for Old Man McMageFace and his Staff of the Magi:

Option 1: Old Man McMageFace can and does defend his belongings

This is what makes sense. He survived adventuring because he had the wherewithal not to do stupid stuff.

Option 2: Old Man McMageFace stopped defending his belongings two years ago

Ask your player if he's sure it's been years since Old Man McMageFace stopped protecting his gear. If "yes", okay, the old wizard lost his magic items three weeks later to some misguided teenagers.

"No problem. We just find those teenagers." Haha, that's funny. They're long since dead, having been killed by the local thieves' guild for the Staff of the Magi. The local thieves' guild, of course, promptly saw a bloody change in management when a vampire took over to get the staff. Then the staff was sold to a prince in a neighboring kingdom, but the caravan was attacked and looted. That was all within the first three months.

To make a long story short, powerful people are willing to kill for such a rare item, so it's been a violent two years. Where's the staff now? Some say it found its way to Athkatla, others say it's at the bottom of the sea, and one guy insists it's now on the moon, with Steve.

Option 3: Old Man McMageFace just lost the ability to defend his belongings five minutes ago

Maybe your player backpedals. "No, actually, I mean he becomes easy to rob when I arrive in Waterdeep."

Fine. Your players' party of nobody's-heard-of-them characters steal an extremely valuable trove of magic from a highly renowned member of the arcane community, including a rare Staff of the Magi. Here's a timeline of events after the robbery:

• 24 hours: this shocking news reaches a Lich. "I've wanted a Staff of the Magi for years!" he mutters.
• 33 hours: The Lich finishes swapping out spells.
• 33 hours, 6 seconds: The level 1 wizard fails a wisdom save against Scrying.
• 33 hours, 12 seconds: The Lich successfully Teleports right next to the party.
• 33 hours, 18 seconds: Veja abaixo.
• 33 hours, 24 seconds: The Lich Mudanças de avião to a demiplane safehouse, Staff of the Magi in hand.

You have a choice as to what spell the Lich casts at the 33 hours, 18 second mark. Enxame de Meteoros is the Lich's smart pick, but using Desejo to duplicate a level 8 casting of comando ("Drop!") will work if you don't actually want to kill the party. In an earlier edit, I said Dominar Pessoa e Power Word Kill, but the Lich would realize that either of those would trigger the Staff of the Magi's spell absorption property.

If your troublesome player is lucky or flagrantly cheats on his dice rolls (don't let him cheat on his dice rolls), but you don't want to kill the entire party, use damage spells with a tight AoE (Aganazzar's Scorcher, Terra em erupção, Passo do Trovão if you want a Con save, Criar Fogueira as a legendary action). He'll drop the staff when he hits zero HP (he also might die; more on that in a moment).

Modify the timeline as needed. Maybe you wait a week for the Lich to learn of the under-guarded staff, so the Problem Player can feel like he got away with it. If you want to build tension, add a couple house rules: First, the wizard gets to know that he just attempted to save against Scrying; second, the rumor the Lich heard was so vague, the Scrying DC starts at 1, but it increases by 1 with each casting (the DC for a Lich would normally be 15 due to the Lich's lack of familiarity with the target). On a successful save, the Lich has to wait 24 hours to try again-- so every day, at the same time of day, your Problem Player is being told to save versus Scrying with no explanation as to why. If you do this, the player might try to argue that the Staff of the Magi absorbs the Scrying. Either tell him it can't ("The targeting is too subtle for you to react to it.") or target one of the wizard's low-wisdom-save compatriots.

Of course, don't feel like it has to be a Lich; lots of level-inappropriate monsters would LOVE to get their hands/claws on a Staff of the Magi, either to use or to sell.

Should you actually do any of the above?

If you wipe out the party out of frustration, it's the Rocks Fall, Everyone Dies trope, but with an absurd amount of embellishment. If the whole thing makes sense as a story, though, that's choices having consequences. YOU are the DM. You can-- and should-- make judgment calls along the way. For example, let's say the Lich knocks the wizard unconscious. Look at your friend. Does he get it? Does he realize that you're not screwing around-- that you'll wreck his day if he tries something as obviously-stupid as trying to transfer magic items from a previous character?

If you need to be mean, the Lich stuffs the wizard's unconscious body (items and all) into a Bag of Holding, then Mudanças de avião away. Hand the player a fresh character sheet; advise him on which rule options you use for rolling a new character. Fun fact: Liches need to devour souls to survive, and that doofus wizard with the Staff of the Magi has one tasty-looking soul.

If you don't need to be mean, the Lich merely plucks every magic item from the wizard's body, then leaves. If the wizard died, it's up to the rest of the party whether they'll raise him. If they do, now the wizard owes all this money, so basically his stupid idea will cost him his share of the loot, perhaps for multiple sessions.

Longer-term consequences

This player just really screwed over his old character by declaring that his brain was wrecked. I mean, jeez. Poor Old Man McMageFace. He's got some kind of neurological condition. Make a running gag out of it, but make the gag more sad than funny.

The characters visit Lord Questgiver's estate. Judging by the decorations, they seem to have arrived during some special occasion. Wait, isn't that Old Man McMageFace? Since he lost his equipment and his marbles, he's been doing prestidigitation at children's birthday parties to make ends meet.

The characters visit a sanitarium. Again, wait, what's Old Man McMageFace doing here, and in a straitjacket no less?

The characters enter the vampire's crypt. SHH! Oh, wait, it's Old Man McMageFace? Working as a janitor?

If you don't think feathers have been sufficiently ruffled by the above, pick a major accomplishment of the previous party. Undo it. "Yeah, without the threat of Old Man McMageFace, The Cult of Cliche Evil reestablished itself and managed to resurrect [insert recurring BBEG from the previous campaign]."

In short, choices have consequences, especially when those choices are so obviously not in the spirit of the game.

The Queen's Crown

Remind the jogador they are playing a personagem.

• Unless you, as DM, reveal things to the personagem -- as you say through legends and songs, etc. -- the entry level personagem would not even know of the existence of this high level wizard.

• O nível de entrada personagem would not know anything about said wizard's possessions, whereabouts, demeanor or current state.

• O nível de entrada personagem does not control other caracteres.

That said, I certainly have the ability to fly to England tomorrow and attempt to steal the Queen's crown, if I so choose. I hear she might occasionally be "naive enough to walk around in [Buckingham Palace] with all her items but without any kind of protection".

Other answers already covered the specific scenario, but I'd like to address one more thing that wasn't mentioned:

Ask him why he wants to do it.

Use this bizarre scenario to know one of your players better.

Honestly, what's the point? What does he expect to accomplish having a Staff of Magi at level 1? More importantly: how does he expect it to make the game any more fun?

If he just wants to cheat and have an advantage over other players or feel overpowered on the encounters, you've learned what kind of player he will be.

Maybe he just doesn't like to play early levels as a Wizard (it happens - a lot) and wants more options, as he is used to an extremely powerful Wizard now - then you might want to use this information and, if other players agree, make the early levels go by faster, for example.

Maybe he has some other bizarre motive - listen to it and, if possible, think on how to solve the problem he wanted to solve by having a staff of magi without giving him a staff of magi at level 1.

That said, obviously don't give him the staff either way.

Just for the sake of completeness of the answer, I'll restate the reasons you can present the player to why this is a bad idea:

1. It breaks the game. Nobody wants to play a broken game, except for the only person getting advantage on it - similar to cheating on any online game, not even people on the cheater's team want to play the game, only the cheater wants.
2. It is flat out against every written rule on creating a character. Heck, by p. 38 of DMG, even if you started a 17th level character in a high magic campaign you would not start with a legendary magic item.
3. There is no way stealing the staff in-game goes unpunished, either by the Wizard himself or by other spellcaster after the Staff or by any of the Wizard's allies or even by the "police" (even a bunch of local guards should be able to handle a level 1 Wizard thief, although it would be harder if he has a Staff of the Magi).
4. If it was so easy to steal from the Wizard, there is no way he would be the first person trying (and succeeding).
5. His character shouldn't have much knowledge about the other character (where he puts his Staff of the Magi while he goes to the bathroom without it, for example, is a very personal information and New-Wiz doesn't know it). In fact, anything that makes it easier to steal the Staff should not be his new-character knowledge, unless it's public - and if it's public we go back to Point 4.

Honestly, I'd stick with Points 1 and 2. Taking into account in-game consequences is already taking this crazy idea too far imho.

Rolling your eyes and saying "Don't be silly" would be one option, but someone who seriously tells you this plan might not pay too much attention to that.

If the character is that naïve then what's stopping it from happening in the intervening years?

One legend could go something like:

There used to be this famous wizard.
Of course he's all washed up now.
Turned to drink.
One day he was walking through town and someone mugged him, took everything.
His confidence just went along with his magic staff.

Alternatively, you could maybe push your starting date forward somewhat and the old characters are long gone.

Between the two extremes (handwaving the staff away as having already been stolen, vs powerful NPC e.g. Lich pursuing the new character), there's are a few other options:

1. The Long Arm of the Law

Sure, old man McMageFace is an easy mark. He's so feeble now that he goes down on the first hit and your player's new character gets all the goodies. However, there are several complications here.

1. testemunhas: did anyone Vejo this mugging take place? If a feeble old man hasn't been mugged already, chances are it's because he sticks to places that are heavily trafficked and other would-be thieves haven't found a good opportunity to make a move without being seen doing so.
2. Old Man McMageFace; incapacitated: does the new player matar him, or just incapacitate him? If they incapacitate him, when he comes to he's going to be distraught at losing all his stuff. A local constable talks to him, and after patiently sitting through a lot of nonsense babble, realizes that Old Man McMageFace was robbed (the huge lump on his head helped get it across too). Now the authorities are aware of the theft and mugging, and likely someone has a clue o que was stolen too, which is something the authorities don't want falling into the wrong hands.
3. Old Man McMageFace; murdered: if the new player leaves the corpse somewhere it can be found, well now the city has a murder on its hands and that's gonna escalate really quickly. Even if he's smart enough to hide or otherwise dispose of the corpse, eventually someone will realize the old coot isn't hanging out at his usual haunts and start investigating.
4. Fondness/familiarity: with either 2 or 3, the local lord/prince/etc happens to have a soft spot in their heart for Old Man McMageFace, who perhaps saved their life when he was younger and less addled, or maybe even just did some magic tricks to help them woo their significant other (magically-enhanced fireworks at the end of a ball, or the like, to make a very memorable evening). After learning of the tragedy that's befallen their friend, there is now a handsome reward posted for the apprehension of the criminals and a bonus for the return of the items. Now every de outros adventurer in the city (and beyond) is looking for a quick and easy buck! Oh, you'll notice I said "criminals", plural. Turns out, the witness(es) saw the whole party in the vicinity of the crime, so now they're todos wanted! Of course if the player is silly or determined enough to do it on his own... well the rest of his party isn't wanted then, but now eles have an easy way to make a buck -- turn on their "friend".

With the city guard on alert and NPC adventurers prowling the streets for a hefty reward, the new character is going to find it nigh impossible to existir in the city; even if he quickly stashes the goodies somewhere outside the city, there's a chance a witness or guard or someone else might still recognize him. Personally I think adventurers would be a better bet; some of the adventurers could be high-enough level to not be obliterated by the OP items, and take them down. If the other party members flee, they get away scot-free, otherwise the adventurers just knock them out too, and the whole party wakes up in individual dungeon cells, separated and item-less.

An important note: if the player does the mugging on his own (i.e. separates from the party and does it without their knowledge), give them time to meet back up and spot his ill-gotten goods. That way when the guards move to apprehend him, they have a chance to react and distance themselves from him (ideally simply holding up their arms and surrendering, saying they didn't know what he'd done and were wondering how he'd gotten those items). I'd hope they'd react similarly if he told them what he was going to do and they actively refused to participate.

2. The Magic Academy

Old Man McMageFace isn't addled at all; in fact, he's still a very powerful and well-respected wizard. His adventuring days behind him, he's taken up the role of Headmaster at a world-renowned magic academy. He spends 90% of his time cloistered in the highest levels of the academy; while not personally protected, the academy itself has guards, wards, traps, and other protections far beyond the ken of a new adventuring party.

Of course that leaves the other 10%, where your player's new character spots him walking the streets (perhaps in search of hidden treasures in the local antiques shop or the like). Yeah, he's naïve enough to wander around with all his crap and no protections. However, the academy would quickly realize that their headmaster is prone to doing something foolish and possibly outright dangerous. As a compromise against not wanting him to leave at all, the new player (with a successful perception check) spots two shiny new -- and in meta terms, unfamiliar -- rings on Headmaster McMageFace's fingers.

It turns out these rings have been custom-designed by the academy to protect their esteemed Headmaster (possibly even with his reluctant help). Both have been designed with a trigger of Headmaster McMageFace taking damage.

The first ring contains a teleporte spell. Even if the new PC manages to knock him out in one hit, *bamf*, his unconscious (or dead) body shows up in the healing ward of the academy (with all the items!).

The second ring contains a (5e adaptation of) Trobriand's Baleful Teleport (City of Splendors: Waterdeep, p. 157; hey, this is a prestigious magic academy we're talking about -- they can make it happen). To adapt to 5e, just make it a WIS or CHA saving throw instead of Will (probably WIS). The ring teleports whoever injured HMMMF -- right into the academy's prison cells. Since the prison cell would be "very familiar" to whoever at the academy imbued the spell into the ring, there would be no saving throw bonus for the miscreant PC.

Now HMMMF is in the healing ward, the miscreant PC is in prison, and it won't take the Assistant Headmaster long to realize how HMMMF got there and send guards down to the prison. Treat prisoner appropriately!

The player has already given you the solution:

The player has told me that, despite having 20 intelligence, his current character is naïve enough to walk around in Waterdeep with all his magic items but without any kind of magical protection.

That by itself isn't a problem, so long as the characterization is consistente, and not something that only applies when future PCs are involved. So accept that the PC really is that naive, and play out the consequences.

Let a low-level but intelligent NPC thief attempt to steal the staff while you're still playing esta campaign. Either the theft is successful, in which case the player has no right to object, since he's already said it would be, or the PC is in fact cautious enough to stop such a theft, and will have no right to object if the same character manages to prevent a similar theft in the next campaign.

Of course, to make your point while avoiding too much ill will, you can allow the PC to recover the staff (probably needing to spend some cash or use up some lesser items in the process; a munchkin tax, as it were). If the PC then starts taking proper precautions, problem solved. If not, you've given him fair warning and he ignored it: the PC now gets a reputation among the local thieves as an easy mark, and the next time the staff isn't so easy to recover.

There are already some very good answers, but I want to add my own piece.

Usually antagonizing players is never a good idea. RPGs are a collaborative effort, and as such you should try to work out things in a way that satisfies everybody.

That being said, in the end it's your game. You are the offering the context for pĺayers to drive the story. Using the same world is an awesome idea, I have done that in the past and it is great for improving the players' sense of immersion in the world when they interact with things they have seen in the past, and with the consequences of the actions of their previous characters.

So my suggestion for you is to roleplay through it. He wants to mug the old character? Fine. He can either:

• Provide a very good explanation as to how he ran into his old character, and how he managed to get possession of his items
• Play through it in session: have him come through where the old character was, find him and try to take his items.

Of course, if the old character is dead, some time may have passed between him dying and the new character running into him... who knows what might happen in that time. The world is full of people who are "less than honorable" and who might... steal... something...

So he wants the equipment. That is a nice plot for an adventure. And yes maybe the wizard is naive. So naive that, in the mean time, he is married to a woman. And that woman isn't so naive. So she won't let her husband parading the streets loaded with magical items and a tag "please take me" attached.

Woman and wizard now have a nice tower heavily guarded. So the low level pc's have a hard time entering. And if they enter, this sweet lady is waiting for them. She beats the hell out of them but she keeps them alive. She sends them home with a mission (probably a geas).

This has two effects.

1. The players learn that high level NPC's are high level for a reason.
2. They have some adventuring to do.

If these powerful artifacts are so unguarded, and his new character knows about it, then surely someone else will have picked up on it also? Seems to me like they will very likely, already have been stolen when this character is ready to do the mugging.

You should still let them try though. Maybe they can even find out who took the items, and the story develops from there?

Strongly urge him agianst such a foolish course of action.

If he persists let him roll play it.

Have an NPC get there first.

You come across the corpse of Rincewind, all his posessions are missing.

If you want to be mean have the guards arrive next and arrest the player character.