The Curse of Arrav
Ritual of the Mahjarrat
The World Wakes
Dishonour among Thieves
After being once again defeated by the power of the Shield of Arrav during Defender of Varrock, Zemouregal departed to his fortress in the Wilderness where he awaited the next Mahjarrat Ritual of Rejuvenation. Shortly before the ritual takes place during Ritual of the Mahjarrat, you infiltrate fortress with the help of a Zarosian Mahjarrat called Wahisietel during The Curse of Arrav and manage to break Zemouregal's control over Arrav. During the ritual itself, Arrav is freed from his curse and allowed to die while Zemouregal leads the other Mahjarrat in attacking Lucien, who is eventually destroyed by the Dragonkin after touching the Stone of Jas.
Zemouregal is next encountered during The World Wakes when you must fight him and the followers of several other gods to prevent them from reaching Guthix. This proves ultimately to be for nothing as he is killed by Sliske.
Following Zamorak's return to Gielinor and defeat in the Battle of Lumbridge, the Zamorakian Mahjarrat, including Zemouregal, attended the "Great Ascension" of Sliske, but were refused access by the Barrows Brothers and forced to flee when attacked by a Dragonkin called Strisath.
During Dishonour among Thieves you must recruit Enakhra and Zemouregal to Zamorak's team for an attempt at stealing the Stone of Jas from Sliske. When doing so, Enakhra has to persuade Zemouregal to follow their master's orders as his loyalty to Zamorak is wavering at this point and he believes he'd be a better leader of the Mahjarrat.