What is Vilemaw?

Shortly put, Vilemaw, "the Spider God", is an eldritch abomination that makes his lair on the Shadow Isles, consuming everything that approaches him, and can grant eternal life to and monstrously shapeshift whoever imbibes his venom extract. So, what is he, exactly?

Firstly, no truly living being exists on the Shadow Isles.

• Yorick, a human, and Maokai, a nature spirit, who are considered alive, are similarly and permanently altered by the Isles, so the Black Mist doesn't differentiate between species. The mist can also corrupt Yordles and the Voidborn. Since nothing on the Isles escapes some level of alteration, in order to reside on them, Vilemaw could't be a pure living being. And yet, he's not undead, either.

• Specifically, he's "...something else entirely, a power that has found a way into the world through the Shadow Isles..." https://www.reddit.com/r/leagueoflegends/comments/51yqct/foundations_team_here_to_answer_your_shadow_isles/d7g23xp/

• A ravenous creature that comes from outside Runeterra sounds suspiciously like one of the Voidborn. Vilemaw even appears similar to Baron Nashor, and shows intelligence, much like other evolved Void-native creatures. However, the Void is not unexplored in-lore or in-game; it's no secret which champions and what things are related to the Void, so why give such a vague answer? If Vilemaw was a mist-corrupted Voidspawn, it'd be directly stated - instead, he's something else entirely, which is fairly similar to Nagakabouros.

Secondly, he is/isn't a god.

• Elise doesn't worship Vilemaw, though her followers think she does. She merely brings him sacrifices to eat, and takes his venom in return. Her deal with him isn't religious in nature, but rather a symbiotic relationship; this is precisely why she's the leader of a cult, not an actual, accepted and organized religion.

• Illaoi, on the other hand, is the high priestess of a real religion and fervently follows the teachings of her god. Nagakabouros isn't just a philosophical concept or a lie told to current and potential worshippers - she is a real entity, and her powers over life and chaos can be manifested. Although her opinion is highly subjective and possibly wrong, Illaoi also outright states that Vilemaw isn't an actual god (https://vignette.wikia.nocookie.net/leagueoflegends/images/2/28/Illaoi.tauntElise01.ogg/revision/latest?cb=20151114232906 ).

• Despite this, Vilemaw and Nagakabouros have a lot in common, based on known depictions of the latter god. Both are tremendous, many-eyed lifeforms with numerous limbs; both are associated with a faction with control over the sea, which are also at constant war; both are also associated with life - Nagakabouros with fulfilled life and survival of the fittest, and Vilemaw with insatiable hunger and cheating death - and both are capable of empowering their agents directly (venom) or indirectly (totem). If Nagakabouros is/isn't an actual god, then, through similarity alone, Vilemaw comes pretty close, too.

Thirdly, and finally, he's completely detached from the rest of Runeterra.

He's neither truly alive, nor undead. He's not a Voidborn. He's very similar to Nagakabouros, but doesn't appear to be a real god (and doesn't even seem to have a real name). And, finally, he's not just a unique monster native to the world, because he first found a way into it when the Ruination struck.

So, what, exactly, is Vilemaw then?