Just as you expected, divination spells are the way to go.

• Augúrio will only tell you whether opening a bag is a good idea or not, but under the circumstances you've provided, that should be all you need. You may need to cast it up to 3 times, though. Note that as soon as you cast it more than once in a day, it becomes extremely unreliable, so a great deal of patience is required if you want to be safe.
• Cidade will let you ask a deity 3 yes or no questions, which can be used to find the artifact by elimination.
• Entre em contato com outro avião gives you 5 yes or no questions, but there's a risk of insanity to go with it - only recommended for high Intelligence characters.
• Adivinhação will let you ask "Which bag is the artifact in?" or better yet, in case this isn't a fair game, "Where is the artifact?"

I would love to play Magey, due more to his DM being a special combination of smart, creative, and the physical manifestation of pure evil. We should work to overcome the challenge in a smart and creative way as well.

Trying to see inside a Bag of Holding

As noted by Miniman, you can use a variety of Divination spells to figure out where the treasure is, and where the liches are. I will give you some clever uses of these spells, and since our DM is smart and creative, he will hopefully not disallow smart and creative ideas from his players.

Augury: Do it in 2 castings, max

The chief issue with this spell is, there is a cumulative 25% chance that you get a random reading if you cast it two or more times, before your next long rest. So really, only the 1st result is reliable. Then we should minimize the number of questions we ask with Augury. The trick is such that when you ask a question, you must always get one new fact for certain that you did not know before.

Arrange the five bags in a line, labeled A, B, C, D, E. Now cast Augury with the following plan of action:

If A contains a Lich, I will open A and B. Otherwise, I will open C.

This plan of action is loaded with these conditionals:

• If A contains a Lich, and B contains the treasure, then we open A and B, getting a Lich and treasure. We should get weal e aflição

• If A contains a Lich, and B contains a Lich, we get two Liches. We should get aflição

• If A contains a Lich, and B contains nothing, we get a Lich and an empty bag. We should get aflição

• If A does not contain a Lich, and C contains the treasure, then we will open C. We should get weal

• If A does not contain a Lich, and C contains a Lich, then we should get aflição

• If A does not contain a Lich, and C contains nothing, then we should get nothing

Then we can get the following information:

• Weal: C has the treasure

• Weal e Ai: B has the treasure

• Ai: B and C do not contain the treasure

• Nada: A does not have a Lich (it can have nothing, or the treasure), and C contains nothing

If you get either "Weal"Ou"Weal e Ai", then game over, you've found the treasure.

Se você pegar Ai, then cast Augury again with A, D, E (if A... open A and D... otherwise E).

• Again, a "Weal"Ou"Weal e Ai" means game over for the second casting.

• But if you get "Ai", then D and E do not contain the treasure either, meaning A must have it (remember, the first casting resulted in knowing B and C not containing the treasure).

• If you get nothing, then you have either A or D as the ones containing the treasure. We know from the first casting that B and C do not have the treasure, and we know from this second result that E contains nothing, and A does not contain a Lich. So you can open A safely. If it's the treasure, game over. If it contains nothing, then D has the treasure.

If you get nothing, then we know C contains nothing and A does not have a Lich. Leverage it by opening A. It will have either nothing or the treasure. If it has the treasure, game over. If it has nothing, then we know both A and C contain nothing. This means the treasure and two Liches will be in B, D, E. We then cast Augury a second time with the following plan of action:

If B has the treasure, I will open B and D. Otherwise, I will open E.

At this point, we know there are only either Liches or treasure in the bags. So the conditionals are:

• If B has the treasure, and D has a Lich, we are getting a Lich and a treasure. We should get Weal e Ai

• If B has a Lich and D has a Lich, then we are opening E, which has the treasure. We should get Weal

• If B has a Lich and D has the treasure, then we are opening E, which has a Lich. We should get Ai

There are no other possible results. This means, on the second casting of Augury, we will know:

• Weal e Ai: B has the treasure

• Weal: E has the treasure

• Ai: D has the treasure

There are plenty of ways to mix this around. Instead of saying "I A has a Lich, I open A" we could easily say "If A has nothing, I open A" and we will be able to construct a new logic path that leads us to the same conclusion-structure, just with different knowledge gains for weal, aflição, weal e aflição, and a non-result.

Of course, the second casting is unreliable already. But we could always use a better spell...

Commune: Do it in 1 casting

Commune is like Augury, in that you want to find the treasure in as few castings as possible. Thankfully, we get three yes-no questions here, and there is no need to be particularly clever.

Arrange the bags in a line and label them A, B, C, D, E.

1. Is the treasure in A, B, or C?

If the answer to 1 is Yes, then we try to narrow it down from A, B, and C. Otherwise, the treasure is either in D or E.

2.i (If Yes to 1): Is the treasure in A or B?

If the answer to 2.i is Yes, then the treasure is either in A or B, but not C. We can use our third question to figure out in which bag is it in.

3. (If Yes to 2): Is the treasure in A?

Obviously, if the answer to 3 is Yes, the treasure is in A. If the answer is No, the treasure is in B.

2.ii (If No to 1): Is the treasure in D?

Going back to the first question, if the answer was No to start with, we only need one additional question. If the answer to 2.ii is Yes, the treasure is in D. Otherwise, the treasure is in E. We can treat the third question as a bonus now, so ask whatever you like!

Contact Other Plane: Do it in 1 casting, but that's obvious

Entre em contato com outro avião will let you ask five questions, and you have 5 bags. You can just directly ask if the treasure is in each bag. Of course, you risk insanity, so just go with Commune or Augury instead. It's risk-free.

Having fun inside the Astral Plane

Plane Shift and Locate Objects

It's boring to be clever! I want explosions!

If the bag is overloaded, pierced, or torn, it ruptures and is destroyed, and its contents are scattered in the Astral Plane.

You can then Plane Shift into the Astral Plane, and use Locate Object (have lots of scrolls!) to find it there.

Parar o tempo

If you can cast Time Stop, you will get four free actions. You can use each of these actions to look in four of the five bags without having the Lich be pulled through, even if you get the bags the Liches are in.

Locate object spell. If they told him all then options the 2nd half of the spell is perfect.

Alternatively, the spell can locate the nearest object of a particular kind, such as a certain kind of apparel, jewelry, furniture, tool, or weapon.

Since he's researched this relic he knows what it is.

BOH does not have the will object thing as the HHH does so id say since an object or lich cannot exit the bag if you close it and hold it its fairly safe look in if its a lich close it stab bag with a dagger try next one itd be too fast for even a contingency spell to work. Or my fave have 4 bottles of holy water and dump them in each bag 1 by one and listen for the dusgrutled shock noises and or nothing lich noise stab the bag and move on but if i were the dm id give them one choice one chance then its the divination to choose

SOLUTION 1: Assuming that Magey knows the artifact is in one of the bags, he can just will the object to his hand as an action. No risk of pulling the wrong thing out.

SOLUTION 2: Send a servant of some kind into each of the bags one-by-one. This might be a humanoid, a beast, something that the character can at least communicate with (Conjure woodland beings? Find Familiar spell?). Take them out after 5 seconds, and ask them what they see in there.

Bonus: Once the Demi-lich bags are deduced, drop those bags of holding into each other (at a safe distance). Gain 2 x 20,000 xp for killing the two demiliches?

SOLUTION 3: IF: a) he doesn't know exactly what the artifact is (just that it's in one of the bags, and that some of the bags contain demiliches); and b) the weight in the artifact bag is known to be less than the weight of a demi-lich (150 pounds?). THEN: fill up each bag with say 400 pounds of rocks. The demi-lich bags will overfill, causing them to rupture, and scatter the demiliches across the astral planes. Then invert the remaining bags.

SOLUTION 4: hire 50 crossbowmen and invert each bag at the bottom of a 70 ft pit. They'd get off at least one volley before the demilich can get close enough to howl. Have to give them some magic weapons or ammo though to overcome immunity... and even then, there's around a 25% chance that 70% the crossbowmen die... but you know... you don't have to pay a dead man.