I think my unwritten "rule" for these is that I need to have seen a monster archetype more than maybe three times in an intentional Halloween season context before they feel really cemented for me as a trope of the holiday, and they can't be three related times either. Fortunately, that's no problem at all for today's entry, which I've now seen plenty more than that both in and out of Halloween!
Again I didn't really want to give these a name that was too unique, though I'm still unsure if just mashing "candle" and "ghast" together is sufficient. It feels like they should probably have a more whimsical name along the lines of Jack O' Lantern or Will O' Wisp, but those have a specific history and reasoning for them, and we're just making this one up as we go!
As titleknown pointed out in the comments, and I should have thought of already, an actual alternate name for the Will O' Wisp itself is "Corpse Candle," so I'm going to say that, while often conflated with the Will O' Wisp, it's also an appropriate alternate name for the Candlegast! While we're at it, I think another nice name might just be Gravewax.
If you know a thing or two about wax, you know that it was traditionally made from animal fat, which of course can include the fat of a human being, and I think it just goes without saying that wax from the dead is exactly what the candlegast needs as a body. Obviously this is usually something a witch, necromancer or other dark magician does to craft themselves a minion, but corpses can also go all waxy on their own, what we call adipocere or "mortuary wax," and candlegasts might just sprout up from cursed enough graves like some sort of surreal mushrooms.
The soul actually animating a corpse candle could have many possible origins. Maybe it's the spirit of the person or animal the wax congealed from. Maybe it's a wandering soul or residual spiritual energy that just needed a body. Once it takes over a candle, however, it becomes an entirely new and distinct type of entity with little or no memory of its previous existence, and if it loses its body again, it may only ever accept another candle as its vessel.
These dripping fiends crave the energy of living bodies, draining away their warmth and with it their vitality, making victims feel frailer and colder the closer they get to the cold, supernatural "flame" of the creature That's more or less how Litwick works in Pokemon, but it isn't even the first time I've seen a life-sucking or cold-producing candle creature in some sort of media, probably because it's such an obvious inversion of something that usually produces light and warmth.
Despite craving energy and warmth however, candlegasts shun natural light as much as most other spirits and specters, and the rays of the sun are the only thing that will melt the corpse candle completely down to a puddle. Otherwise, they have full control over their own physical consistency, liquefying and solidifying as needed to sneak around their crypts and cellars like some sinister, luminous amoeba.
The more powerful a Candlegast, the more it can control than just a stick of wax. It's the older, better-fed specimens that employ candlesticks, lanterns or chandeliers as bodies, while especially powerful Candlegasts can take almost any object as a vessel, so long as their wax is in contact with it. Some may puppeteer an entire dead body, becoming its new head or face or more discreetly carried around by the corpse like an ordinary candle. Others might manipulate a larger piece of furniture, a dead tree or even the surrounding architecture, and hundreds or thousands together could control an entire old house as well as any infestation of poltergeists. The older and stronger they get, the more intelligent as well, and an elderly Corpse Candle might ascend from skulking dungeon minion material to the very master of their own haunted house, attracting other creatures of darkness and living dead but also cold-loving monsters as minions.