Last night, three of my players’ 14th-level PCs went up against a death tyrant in its lair.
They quickly discovered its weakness. Without the antimagic cone that a living beholder would have, the death tyrant had no way to get the PCs to stop turtling inside the warlock’s sphere of darkness, from which she could snipe the tyrant with impunity.
I had given the tyrant the Alert feat, so the warlock didn’t get advantage on her attacks, but with three attacks per round and a minimum of 11 damage with each eldritch blast thanks to Agonizing Blast and Hexblade’s Curse, she was trashing the tyrant while it could do nothing in return.
Why? Because all the tyrant really has is its eye rays, all of which require line of sight. Its three lair actions weren’t of much use either.
Yes, it had a horde of 40 zombies as backup, but the cleric happened to position his dawn spell by the tunnels the zombies came from. The zombies didn’t stand a chance.
What was supposed to be the epic big boss fight of the dungeon turned into a pretty one-sided affair. Halaster wasn’t happy so he threw in another monster with blindsight to shake things up. The PCs still won.
Have you got a similar story where the PCs happen to stumble on a boss monster’s weakness that turns what should be a deadly encounter into a triviality? And how did you deal with it?
(In this context, I appreciate having the excuse of a god-like antagonistic NPC who can interfere to raise [or lower] the difficulty on a whim.)