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It shouldn't be invisibile in the first place.
Invincible
The Terrasque is invincible. It's immortal, cannot be killed, and would take a party of 20 level 20 players to knock unconscious
5e Tarrasque: "You guys are getting regeneration?"
Why not?
Well, of course it depends on the type of game and context, it just sounds a bit like an uninteresting tool, to just have a just invincible NPCs. I think special weaknesses make every NPC more interesting. Give the NPC a weakness (eg. Superman hs cryptonite or Lord Voldemord has Horcruxes).
OP said it was invincible, not that it didn't have a special weakness - you're assuming extra context that wasn't mentioned, when, as you say, the type of game, in game context, and additional conditions like a special weakness can make it entirely appropriate to have an invincible NPC.
Im curious too. Why shouldnt a boss be invisible... sorry, invincible ? Especially when you have no clue of the context except that it went away and the players survived ?