HUBRIS. This classical Greek word is usually translated as pride, but it's more than just regular "Proud of achievements-" it's pride so strong that you think yourself above the very gods. Appropriate for the godless yugoloth! And it certainly plays into the whole "Divinity Leech" aspect, for whatever value that has for them. (I'm afraid I've forgotten the details of what exactly they extract or if it's ever explained how they use the results, and a quick search isn't turning anything up right now.) But it also seems to tie into his belief that he is greater than the Demented.
Whatever the Ebon is, he has an absurd degree of control (above and beyond that granted to the Oinoloth) over the stuff that 'loths are made of- his creation of the Astraloth is major proof, but at the spot we're at now there's also modification of mezzoloths for use in the inner planes going on. There may be a hint of his nature in the words Shylara uses in transforming Mezzoloths: "And you I sacrifice on the altar of our purity." There are perhaps also clues in Vorkannis's interactions with the Architect- he seems to flinch ever so slightly in that interaction.
So: He's been working on this plan for a VERY long time- so long that he was responsible for Shylara's ascension… and for Shemeska's far, FAR longer ago fall to mezzoloth status and her struggle all the way back up to Arcanaloth. This extreme age and ridiculous foresight suggests he's something adjacent to the Baern. Here's my current theory: He WAS a baern, in the very earliest days of reality. He was a baern who was broken down and used to CREATE a prototype of the Yugoloth. (Even if it's not true, I think it's a good plot hook for my own later use.)
What he's up to? I'm not sure. I don't actually have any idea what the Oblivion Compass's deal is, or where it's counting to.
All that said: I do not think that the Ashsinger or Apomps are allies of Vorkannis. [This was related to a former player in my games own speculation] Apomps as a former associate, perhaps. But not an ally. The Ashsinger, I'm still unclear- but I think that it's perhaps related to the Far Realm somehow, or something else "outside" reality. It seems almost unrelated to the Ebon… but maybe not to whatever the Oblivion Compass counts down toward. Tollysalmon (and the swiftly deceased Alex, by extension) seem to be definitely tied to the Far Realms. Whatever the Ashsinger is, it seems like it can mess with time (Leobtav in the past talking to Guvners), leaves both Law and Chaos susceptible to its naughty word, and is awful enough that the Yugoloths revered it. A baern that went "outside" maybe?
Hm… something else that caught my attention: the mention that there were only four remaining "hosts" for the Demented. Taba, Xenghara, Helekanalaith, and Larsdana. It seems like the Altraloths are sort of proxies (in the non-technical sense) for the Demented, and suggests to me that the entire massive battle for Khin-Oin was almost as much about cleaning house and knocking off their proxies as it was about getting access to the Fleshforges of the Wasting Tower. BUT. It's also explicitly about wanting the Yugoloth to be a closed circle, with no interference from the Night Hags.
Then there's the Original Jester. He seems to be LE, and to have a deep disdain for Asmodeus and the Devils. Possibly some variation on an ancient Baatorian? He "infects" Clueless's thoughts in some fashion, which seems appropriate. Green Marvent I can't get a read on, but I'm actually guessing not a proxy/avatar/chosen? That seems too... easy.