Ostegoth the Merchant has brought Death to the Icy Veil where he recounts the history of Argul, revealing a added detail about the story of the former dead king: Argul summoned a creature known as Frostbane from the Abyss that was used as a immense source of power.
When the Dead King imprisoned Argul he cast out his tomb so as to cut Argul off from his power, and while Argul may no longer be a threat Frostbane and the threat he presents is alive and well and must be destroyed hence why Death has been called to arms. After a long chase through the tomb the Abyssal Creature reveals that Death is wasting his time.
Frostbane's soul is not one that can be cast into the Well of Souls, his is a incarnation of the Abyss itself. None the less after brutal fight with the dragon creature Death succeeds in killing it and is given the promised reward by Ostegoth, the Frostforged Twins.