He frequently procs some ability that says he "Heals himself for 9113 HP through Unwilling Devotion".
It happens right after an emote that says "He looks Compelled".
He procs it quite often so it takes forever to bring him down.
https://lucy.allakhazam.com/spell.html?id=23490&source=Live
It seems to be a single target Lifetap DOT.
Edited, Jun 23rd 2023 7:58am by tounces