All by himself, he sits on his throne.
This skull is found in a lonely chair in the Deep Cavern. This area can be reached through the Dark Fissure in the Crumbling Depths in Deepholm. The skull can be interacted with while wearing a 
[Shadoweave Mask], in order to spawn the next Inconspicuous Note.
Patch changes[]
Patch 7.3.0 (2017-08-29): Added.