Some ideas about Vilemaw (a lot of text)
Note: By virtue of being such a minor and nondescript character, any ideas related to Vilemaw will have to indulge in a large amount of speculation. I, myself, am going to be speculating a rather sizable amount in this thread.
Not much is known about Vilemaw, other than that he is a tremendous entity worthy of Lovecraftian myth. A mysterious being that found its way into Runeterra via the Shadow Isles, he is the Great Old One who acts as Elise's supernatural patron, being both the source of her transformation and immortality. He also has the dubious honor of featuring in the game as the boss of Twisted Treeline, but that's not as important. Suffice to say, he's a less-than-significant character who I feel could nevertheless benefit from some attention. I'll try to go over some of my ideas for this enigmatic creature in this thread, and I'll start by looking at what Vilemaw is and where he could come from.
#Vilemaw as a spirit god? This is perhaps the most obvious way to explain Vilemaw. Being a powerful animalistic entity that transforms its follower(s) into creatures that resemble it, Vilemaw is an obvious fit for a spirit god of the Shadow Isles. In fact, I would draw comparisons between him and Volibear, as both are scarred, monstrous beings of inhuman intellect served by shapeshifting followers. I will also mention that he is described as a "demigod" (as the Freljordian spirit god siblings are), and that Nunu speaks of a "spider" when reciting one of the songs Laika taught to him, which to me seems to link him to the known spirit gods; the quote can be found here.
Since the forms of spirit gods are warped by the domains humans ascribe to them, Vilemaw would require an explanation as to what, exactly, he is a spirit god of (Ornn represents fire and creation, Anivia winter and cycles, etc.). Possibly, going by his dark theme, Vilemaw is supposed to be a "dark" spirit god, i.e. a spirit god with powers over less-than-positive domains; since he is constantly hungry and resides on the Shadow Isles, I would give him such domains as corruption, or perhaps decadence, suffering, decay, stagnation, overindulgence, gluttony, etc. All these (and others) are appropriate for both him and Elise.
Now, since I've already mentioned a spirit god can "change their nature" depending on the perception of them (or be known under multiple guises to different peoples), it's actually possible that Vilemaw existed before the Ruination, but in a different form; in this case, "Vilemaw" would be his new identity and the new domains he has control over, whereas his old form was something he no longer represents. Since the Blessed Isles were a religious, scholarly society in tune with the spirit world and its inhabitants, Vilemaw may have existed as a more benevolent or neutral deity. Culturally, spiders are associated with some interesting ideas, like time, fate, creation, trickery, cunning, or knowledge; any one, two, or more of these attributes could've been embodied by Vilemaw in his previous incarnation, and I would say they certainly would've been fitting.
These aspects could've been corrupted by the Ruination, or perhaps Vilemaw changed his priorities. His scarred, hungry self mirrors Elise in a way: like she crossed the event horizon when she killed her jealous husband and decided to sacrifice what she had left to restore her lost beauty, so, too, could Vilemaw have crossed one when the Blessed Isles fell to the Ruined King's hubris. Perhaps he also seeks to heal his scars and restore his former, beautiful self?
#Vilemaw as a demon? On the other side of the spirit spectrum, there are the demons: ravenous spiritual parasites that feed on something specific produced by humans and other sentient beings. Just as Tahm Kench feeds on mental anguish and Evelynn feeds on physical suffering, or Nocturne feeds on mortal terror, Vilemaw also kills and devours his victims. It is not impossible to imagine that demons can empower human servants just like spirit gods do; therefore, there's no reason for Vilemaw not to be able to transform Elise with his magical powers. Cults to demons are also not unknown: Evelynn is shown to have mortal followers in Vayne's story.
Just like Nocturne was born from shadow magic, and Evelynn was born from the Rune Wars, Vilemaw was born from the Ruination - yet another magical catastrophe that called forth a powerful extra-dimensional entity into the material world. The question becomes what Vilemaw specifically feeds on, but, since Elise's cult is of a religious nature, it's possible he's empowered by the worship of his victims; the more belief his dinner has in him, the better. In this sense, he could be a sort of "demon god": a twisted parallel to spirits like Anivia or Janna.
#Vilemaw as a celestial? This one has less information to back it up, but I might as well mention it. We know Aspects literally embody aspects of existence, and, as such, may represent any given thing in any one, more, or all of its forms. Due to the significance of celestial symbolism on the Blessed Isles, Targon's presence on them, and the real-world association of spiders with cosmic elements in certain cultures, as well as the fact that Aspects can and do fall or are even erased, Vilemaw could very well be a grotesque Aspect, rogue celestial, or other kind of inhabitant of the high realms stranded, cast down, or intentionally descended to earth. Since he came with the Ruination, he could also be associated with it.
I will also mention that the Ascended who became the Darkin were empowered by celestial magic, and learned to be quite skilled in fleshwarping; this could explain both Vilemaw's bizarre looks and Elise's transformation, though I'd prefer him not to be another member of the Darkin.
#Vilemaw as a Voidborn? A strange twist, but also worth noting.
Taking his appetite and physical resemblance to a Voidborn into consideration, and also recalling Riot's statement that Voidborn can indeed be corrupted by the Black Mist, it's possible Vilemaw is the result of a Void creature being killed and brought back by the Ruination. If this is true, then his level of power and intelligence would put him somewhere near Vel'Koz and Cho'Gath; he's also quite similar to the latter, and possesses the psychic abilities of the former. Additionally, Vilemaw is also described as "undead". While this is not my preferred method of explaining him, I'll mention it simply to cover all bases.
#Vilemaw as something else? Eldritch creatures don't necessarily originate from the spirit world or the Void in Runeterra. Beings such as those from the Swimming City and Nagakabouros are suitably alien in a way unique to themselves; it's possible Vilemaw is one of these beings, though I'd be very sceptical to put him anywhere near Nagakabouros who is, for all intents and purposes, a fundamental and constant element of the universe.
There is also mention of beings such as "the Weaver" in Taliyah's lore; while I am, by no means, suggesting that they have something to do with Vilemaw, I'll bring them up as an example just in case.
Perhaps more important than his origin and affiliations is Vilemaw's nature in the story, and how he can impact the world. I'll now go over his worldly influence and possible uses in the lore. #The religion of Vilemaw Obviously, Vilemaw has a cult, which is currently led by Elise. Composed of members of the Black Rose and having been active since its inception for several centuries, the cult exists to sustain Elise's youth and give Vilemaw something to feed upon. This is fine and well, but I believe there are other ways to logically take the cult theme and expand on it.
Going by the assumption that Elise's lore will remain the same by the time Riot decides to revisit Vilemaw (if they decide to do that), it is both in Elise and Vilemaw's best interests for the cult to be more prevalent, and this firstly involves expanding it from a shadowy sect into an actual religion. Whether it is Noxus-centric or not is of little concern; Elise can just as easily focus solely on Noxus Prime as she can on small villages, the entire empire, or even other empires. This would also benefit LeBlanc, since the cult's growing influence could undermine that of the Trifarix and its hold over the masses of Noxus.
Think of how many of the wretched and dispossessed would jump at the promise of a new faith, led by a charismatic and enchanting (literally and metaphorically) woman as Elise? The disheartened and powerless are plenty in Noxus, as evidenced by their overall poor living conditions, and many more await beyond the Prime's walls. It should be fairly easy to introduce them to a new promise - a "paradise", where all their worries are taken away. Elise already has significant ovetones of hiding the cult's secret purpose under a mask of good intentions ("I will take them all to a better place."), so pretending to be a benign religion dedicated to the reverence of a merciful "God" would be an easy way to take the cult. In fact, there are already implications that Elise is spreading her influence across Noxus, as the port city of Basilich has recently become somewhat famous for its cobweb paintings. With the resources at her disposal, it wouldn't be impossible for Elise to fund the construction of actual places of worship, either; in a way, Vilemaw would be spreading his web over Valoran.
#Vilemaw in a story It's a bit difficult to make a story with Vilemaw that isn't plain horror, but if that's the case, might as well go all the way with it, right?
The idea of an outwardly safe religion that has dark secrets hidden beneath its exterior is fairly classic and the basis of many a religious horror. Elise's brainwashing powers basically ensure that fanatical, knife-wielding worshipers are a given in such a story, which can be a fun challenge for whichever character is playing the role of the protagonist. Coupled with copious amounts of spiders and creative use of them (Elise has already demonstrated that she can make them mimic people), it's an arachnophobe's worst nightmare. Put someone righteous against them (like Lucian or Vayne, for a given meaning of "righteous") and call it a day.
More interestingly, a story set before or after Vilemaw came to the Shadow Isles could go into more detail on the various afterlives, the spirit world, Vilemaw, and the Isles themselves. In that case, it'd be a less grounded and more mythical piece of lore, but one that could then tie into the greater scheme of things.
Finally, I would like to discuss the possible "motivations" of this nebulous figure. #Vilemaw's agenda I'll admit, it's practically pointless to speculate about this right now, but I'm going to do it anyway.
Apart from being background material for a single champion, the nature of Vilemaw and the extent of his powers offer a number of interesting possibilities for several different parts of the lore. If he is indeed a spirit god or demon, then that means that the presence of both groups extends even to the Shadow Isles (conflict with other powerful spirits like the Ruined King and Maokai is possible); this also means that the exact nature of spirit gods and demons, as well as the spirit world itself, can be further explored in a story where Vilemaw is of importance. His relationships with the other powerful spirits show some promise as well, and I'm sure that the recently introduced concept of afterlives can be made to fit into the lore of a god who gives immortality.
Vilemaw's own goals are what interests me more, though. He's already shown intelligence and will definitely show it if they turn him into a spirit; this means that he likely has a purpose beyond bloating himself with sacrifices. Even a demon like Evelynn implies that she wants to instigate another Rune War, so regardless of scope Vilemaw's purpose would at least be more than what he has right now. Elise specifically strikes me as someone ambitious, so merely staying alive is also not the only thing she should be interested in. Overall, I'd say Vilemaw is likely stuffing himself up for something, and I'd like to know what exactly that something is.
With all this talk of "Spider God", it makes me wonder if he's actually interested in becoming an actual one - or just accumulating more power than he has right now. You could take the "spreading his web" idea and replace "Valoran" with something bigger - the other continents, or even the world. It is not exactly out of the realm of possibility that spirits can be as ambitious as mortals, and the same goes for whatever other kind of power Vilemaw is.
That is about everything I could remember to mention about Vilemaw. I tried to use as much of what we have right now as I possible, but there's always the fact that Riot could just decide to come up with something brand new, never expand on him, or even scrap him from the story completely. In any case, I'm interested to see what they'll come up with.