>>96311236
I can share my experience with Dming a BBEG in an ongoing campaign. It's in 3.5 D&D.

I pitched it to them as a Dungeon Crawling thing that would evolve into something more plot-oriented.. There was one big dungeon, a couple of villages, a great city and Forest and Mountains full of things to discover, fight and be destroyed by. I'll just focus on what they did in the Dungeon since that's what's relevant:

It was the, well, dungeons of a ruined castle, under which there was a complex of catacombs that sprawled underground. From the first floor, it was constantly decorated with Tapestries, Engravings and Statues that depicted 2 main characters: the mythological First Warrior and another, real person: Volkmar, the shining paragon of what a Knight could be. He was famous for three things: His conquest of the North, his magical sword and his slaying of the only dragon the world has ever seen.
As they kept exploring the catacombs they found more and more weird stuff: Living statues created to keep intruders from advancing, riddles, tests of character, corpses of unlucky adventurers and necromancers, Demons and Undead. They kept learning more and more about Volkmar, until, at the last floor, they learned the the entire complex was a mausleum for him, his men and his family.

At the end of the last floor, they found a great passage that went further and further underground, at the ent of whitch they found his tomb, that was contained inside the skull of the dragon he killed. The tomb was open.

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