Cinematic Dungeon Design #2: Speak Once.

I'm all for your BBEG monologues, okay? They're great, for certain campaigns. But if you want to create something memorable and terrifying, have your monster speak sparingly.

You're going through a dungeon. As expected, you scrap with evil minions, barely dodge some traps, and step over so many unidentified bodies.

But then you come to an ancient door. It's big. And it's been sealed for a while.

Once you open it, you see a creature, waiting for you in the mist and silence.

It's got that 'final boss' aura. It opens its mouth and, instead of a monologue, it says two words: your name.

Those are the kinds of monsters that stick with your players. They don't need to explain themselves; they've done that already, in the chaos they've shown your players throughout your entire campaign.