Zumog Phoom is a powerful Orsimer necromancer serving under Euraxia Tharn. Initially, he acts as Euraxia's advisor and strategist, and provides her with large amounts of reinforcements in the form of necromancers and undead soldiers. However, his main aim is to put Cadwell the Betrayer back together and gain favor with the dragons. He and his necromancers abandon Euraxia shortly before her death.
Zumog briefly appears to turn Euraxian soldiers in Riverhold into undead, and casts a spell to continue to create more undead. This requires the Vestige and Zamarak to activate the blessed Moon Pillars to prevent the undead from rising and overrunning the city.
Zumog traps the Vestige in the Chapel of the Guiding Moons of Rimmen Palace. Later, they are found performing a ritual on the soul shriven Cadwell to restore the Betrayer.
When the Vestige reaches his lair, he attacks and summons multiple undead throughout the fight, including a dragon skeleton. He transforms himself into a Bone Colossus later in the fight, but is ultimately defeated. However, he succeeds in restoring the Betrayer.