1 Eleven: So we have the two halves of the golden key. I suppose we put them together and they magically combine into one key?
2 Lucas: They do, but don’t do that!
2 Eleven: Instant death?
3 Lucas: You’re learning, but no. It calls the soul of the demi-lich Acererak back from whatever strange planes of existence, unknown to even the wisest sages, that he was roaming.
4 Lucas: To prepare for our imminent arrival in his crypt as would-be vanquishers!
4 Dustin: That’s pretty optimistic.
4 Lucas: He is insane.

The backstory of the Tomb of Horrors is that after many centuries of magically extended life the evil magic-user/cleric Acererak took the necessary evil steps to preserve his life force as an undead lich. In this form he continued for hundreds more years, constructing his infamous Tomb to store his treasures and confound the unwary and send them to their doom. But eventually time defeated even his undead physical form, which decayed to naught but a skull set with jewelled teeth and a pile of bone dust. Meanwhile his wretched soul detached itself and wandered multiple other planes of existence. The adventure doesn't explain exactly why he did this, but it's reasonable to assume in search of further power and means to prolong his existence.