From publisher blurb:
Evil lies trapped within the Crypta Hereticarum—the Vault of Heresies. The Clergy believes that true evil cannot be destroyed, and that killing an unholy disciple or destroying a cursed item will only free its malevolence to wreak havoc elsewhere in the world. For centuries, whenever the priests and godhands of the Clergy collected dangerous items, captured evil acolytes, or rescued afflicted innocents whose curses they could not break, they placed them in the vault, built beneath the desolate isle of Odiem, off the coast of the city Sid Minos.
After the Great Malice, the Clergy fell into disarray for years, and those responsible for maintaining the vault had more pressing issues. They sealed it, tried to erase knowledge of it, and used their divine power to compel all those who had drowned in the rocky seas nearby to rise up and slay any intruders.
Many still tried to plunder its treasures—indeed, in the default version of events, it’s already picked clean by the time the PCs arrive in Adventure Four, Always on Time. But these dungeon delvers are drawn not only by the promise of rare riches and powerful magic, but by the mysterious temptation of the vault’s greatest prisoner, Ashima-Shimtu.