Dark Sepulchure is the Necrotic Blade of Doom taking over Sepulchure's Dead Body.
After defeating Zombie King Alteon, the players see Sepulchure about to deal the final blow to Gravelyn. Sepulchure finds himself unable to kill his own daughter.
Just then, Drakath appears and strikes at Gravelyn only for Sepulchure to take the hit. Gravelyn unleashes her full light powers to drive away Drakath. The Doom Blade then takes control of Sepulchure's body transforming it into Dark Sepulchure in order to finish the job. The players end up fighting Dark Sepulchure and managed to defeat him.
Artix tells the players that a shadow creature can only be slain by the Blinding Light of Destiny as Dark Sepulchure gets up behind them.
Gravelyn leaves her fight with Drakath and grabs the Doom Blade which she purifies into a weapon that destroys Dark Sepulchure and restores reality back to normal.