Essence of the Executioner
As a quest objective
To obtain the hidden appearance for the affliction warlock artifact, you must first be level 110 and have a minimum of Artifact Knowledge Rank 5 before the will have a chance to drop from eredar mobs in the Broken Isles. Azsuna is a good zone to farm for the grimoire due to the concentration of eredar mobs in Faronaar.
Once you have the grimoire and picked up its quest, be on the lookout for any "DANGER"-type world quests where the rare elite mob could reasonably be presumed to have a humanoid skull (such as a murloc versus a wolf). Due to the rarity of "DANGER"-type world quests and, on top of that, mobs that are eligible, this will take some time to complete. IMPORTANT: Do not do the related world quests while in a raid group!
After finally collecting the required 13 , take the portal from Stormwind City / Orgrimmar to the Blasted Lands. Fly west to Morgan's Plot near Karazhan in Deadwind Pass and travel down the stairs of the Forgotten Crypt until you have reached the Executioner's Altar in the Well of the Forgotten. Turn in the first quest and pick up the final quest to perform the ritual, upon which Sataiel's soul will manifest and fight you. Kill her and loot the Essence of the Executioner.
- Patch 7.0.3 (2016-07-19): Added.