This is too complex to be playable but I was thinking about how a dungeon this insane would come to exist in a fantasy setting..

The PCs come to a specific tomb in the Hills of the Dead, one of many. Maybe a rival flees to hide in the tomb, maybe a local noble hires the party to retrieve a relic from some remote branch of her family tree. There are rumors that the nobles practiced necromancy thousands of years ago, but nobody pays them any mind.

The PCs recover a map. An old vellum scroll from the dessicated body of an adventurer, a Raiders of the Lost Ark-style model on the floor, a lattice of dark angular rooms connected by minute lines barely visible in the middle of a crystal obelisk, a dark orb that imprints passages into the mind of whoever touches, possibly replacing other knowledge. The map; describing a small, simple tomb, turns out to be wholly inadequate.

The wizard/bard/lore master deciphers some of the glyphs on the wall. A Priest-King is laid to rest in the sacred hills, his entire household sacrificed with him. As they venture deeper, the writing becomes more degenerate and eventually disappears. Dwarfs, those raised underground, and skilled masons note that the tunnels that were once perfectly straight passages that ran in mystic alignment with constellations and ley lines have become well crafted, then of average workmanship, then just plain shoddy and dug by brute force rather than skill and dammit I could have done better my first day as an apprentice.

At the deepest point, the old Priest-King, now Lich, still directs his minions to dig. Very few of his original servitors are still "alive"- some were killed in accidents, but most were killed by adventurers or by other forces the Priest-King encountered in his delvings. Maybe he dug into a Dwarven mining shaft or a pocket of Myconids. Maybe the lich attracted the attention of a band of magma elementals trying to force their way to the surface. Maybe there are thousands of years of defeated, undead adventurers. Maybe everything and more over god knows how many centuries of digging. Regardless, the lich has Stone Golems and a pretty diverse array of undead. The PCs fight them and barely win; it took a few sessions and the players feel a good sense of accomplishment.

As they leave the tomb, the bard/wizard/lore master looks at the massive, weathered inscription. It's much easier to decipher with the experience they've gained in the tomb. The inscription says the Priest-King was interred 823 years ago, but as they walk back to collect their reward, he can't help but notice this this is one of the younger tombs. One Priest-King died 1,759 years ago, one 4,831, one 2,197. The size of the huge piles of dirt and broken rock sprawling out of the entrances seem to correspond with the ages, and a particularly observant party member swears the piles are larger then when they entered the tomb.