Acererak is a powerful demilich and creator of the Tomb of Horrors.
Nothing is known of Acererak's life and undeath prior to the creation of the Tomb of Horrors. A devotee of Nerull, he constructed the tomb as a massive altar to the god of death and weaved magical spells into the complex to funnel the souls of all those slain within directly into himself. In order to ensure a steady flow of souls, he spread stories and rumors of the vast wealth that lay within in order to entice adventurers and treasure hunters to their deaths. It is generally believed that he sought to use their power to become a demigod, or possibly even a god. It is not known how Nerull's death in the Deity War affected Acererak's plans.
In 2959, the Challengers of the Unspeakable entered the Tomb of Horrors and destroyed Acererak, taking his soul gems to utilize in a ritual to close a portal into the Far Realm. The soul gems were eventually stolen by agents of the sorceress Theriana, but after her defeat in Estorium it is unknown what happened to the soul gems.
In 2968, resurrection spells began to fail across Syra. Leardon Sunflare's investigations led him to believe there was some sort of link with the Tomb of Horrors and sent asked fellow Brightstone Hero Flynn Cupsigh and his companions to investigate. It soon became clear that Acererak had returned. While exploring the tomb, they freed an efreet who granted them three wishes in exchange. With one of their wishes, they asked for knowledge of how Acererak could be permanently destroyed. He replied, "In the jaws of darkness, at the hour of ascension, only a great sacrifice of souls can forever destroy Acererak." At the heart of the tomb they confronted and defeated the demlich once again. Though they knew this defeat was not permanent, they nevertheless delivered his soul gems to Leardon.