Qualquer ataque irá revelar a posição do chefe:
O chefe pode ficar invisível somente se ele não atacar. Da página do PHB (p 194-195)
Unseen Attackers and Targets
Combatants often try to escape their foes’ notice by hiding, casting the invisibility spell, or lurking in darkness.
When you attack a target that you can’t see, you have disadvantage on the attack roll. This is true whether you’re guessing the target’s location or you’re targeting a creature you can hear but not see. If the target isn’t in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target’s location correctly.
When a creature can’t see you, you have advantage on attack rolls against it.
If you are hidden—both unseen and unheard—when you make an attack, you give away your location when the attack hits or misses.
O feitiço da invisibilidade também aponta isso:
A creature you touch becomes invisible until the spell ends. Anything the target is wearing or carrying is invisible as long as it is on the target's person. The spell ends for a target that attacks or casts a spell.
Os próprios PJs ainda podem tentar localizar o chefe atacando áreas onde eles acham que ele estará e poderiam fazer efeitos AOE para tentar afetá-lo sem saber sua localização exata (como Faerie Fire ). O também pode tentar agarrá-lo, ele se revela com um ataque para impedi-lo de se mover, mesmo que o chefe tente ficar invisível.