Nobody actually knows. An otherworldly anomaly from outer space maybe? It first starts corrupting places with rifts then when it's corrupted severely enough , it can consume that place within the abyss(no clean/even less pure place can be consumed within, it has to be severely corrupted) and it not only consumes places from earth but from other unknown planets' territories also.
In the novel there's a vivid explanation/actually kind of an assumption of it's nature with a comparison. I'll explain a bit more when that ch comes

Ok so I've read the novel cus I couldn't wait lol it's so fckin good. The ending is kinda abrupt(?) I would say? But also not really? Anyways read the novel if yall got time.
And for the nosy people here in this ch(ch 11) the whole subway station shown to us is through mc's eyes and an complete illusion. It's a fckin enormous monster sent by abyss to eliminate mc(the anomaly which can purify the bullshi it creates, yup the abyss thingy, inside which mc & ml are rn, has active consciousness)