Speak to Kerr Wanderroot at -1786, -599, -1068 at the Sanctuary of Mourning to begin this quest.
You will need to defend Kerr while he performs his ritual. Speak to him again when you are ready to begin.
Kerr will be attacked by stele corruptors (level 92v). The attacks will come more quickly as the ritual progresses. There were about 5-8 corruptors total that attacked.
If Kerr dies you will have to re-start the ritual from the beginning. Wait for him to re-spawn to tell him you're ready to go again.
If Kerr finishes the ritual wait for him to sit back down at the camp to speak to him again.
Return to Brint Thornwood at -2162, -650, -458 .
Speak to Leif in the Sanctuary of Waning at -2401, -675, -179 to complete the quest.
