From the introduction:
In times ancient, an infamous paladin-artificer named Khaldun unearthed a cursed tome bound in the flesh of the first medusa. Driven to madness by its secrets, he used the necromantic magic contained within to call forth an army of dread warriors from the Netherworld.
Though his assault was devastating, he was ultimately unsuccessful, and the mightiest crusaders of the realm banished Khaldun to the Netherworld, where he would rot for eternity at the bottom of the River Dis.
Unfortunately, rather than perishing in that bleak plane, Khaldun escaped his silt grave
and thrived, amassing a legion of undead followers and erecting a Tomb in which he could perfect his mechanical craft without interruption. It was not long before he shed his body and became a demilich.
After several centuries of scheming, the undead warriors under the command of the 'Iron Lich' have burst free of the Netherworld, laying waste to not only the mortal kingdoms that exiled him, but also the exalted domains of the gods themselves.
The crusaders' quest is simple – stead the Tomb and destroy the Iron Lich.