Rift Quest:The Grave of Amunet  

The grave of Amunet can be found at (1165,2347). Interact with the grave to summon a Corrupted Lich of Amunet. When its health drops below 50% use the Grimoire of Ice and Storm. It will despawn and another will take its place. Repeat this process and the third lich will be immune to the Grimoire and can be killed normally.

Speak with Spirit of Amunet to obtain the Amulet of Eternal Imprisonment to complete the quest.