Oerth is a world in the D&D multiverse, mentioned briefly in the context of Acererak’s origins.
Connection to Acererak
Ancient texts assert that the powerful lich Acererak comes from a world called Oerth. While much of Acererak’s past has been forgotten, this detail about his origins remains recorded in old writings. Despite coming from Oerth, Acererak has since traveled extensively throughout the planes and lived on many different worlds while building his numerous deadly tombs.
Adventure Context
Oerth is also referenced as an alternative starting location for the adventure. The Free City of Greyhawk on Oerth is suggested as one possible large settlement where the adventure could begin (instead of the default starting point of Baldur’s Gate on Toril in the Forgotten Realms).