Oh no, please, stop with all these bosses ideas you're giving me... WINK
"Yes, I'd hate to have more awesome boss fights... WINK WINK."
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?"
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 ?