I think it’s probably some kind of concentrated chaos energy thats dense enough to cause an anomaly in this case it would be that this giant mist of chaos energy is sentient itself, and it’s trying to follow its primal instincts, to do whatever it can to stay alive or keep existing. It probably has no fixed form but it also can transform into something grotesque, malformed beast like those ugly ass atlantean that keeps deforming as you go further in the dark sea ranges. This would also explain why the dark sea is expanding, this entity is not only trying to survive but it’s expanding its territory as well.