You might like getting some inspiration from the main antagonists of the Kirby video game series: Dark Matter, an evil alien race of usually eyeball-shaped (but not always) eldritch abominations which reproduces asexually and probably feeds on negative emotions. (0, which is the being that creates and controls all Dark Matter, is probably weak to positive emotions since the love-love stick is one of the only things that can harm it.The nightmares in Nightmare in Dreamland, were created by Nightmare, which is almost certainly a form of Dark Matter, judging by its similarities to Dark Mind in the Amazing Mirror.
Not Arcane. Definitely not Arcane. Should be mind/dark. Eldritch abominations would never make sense in the arcane school. If arcane school found an eldritch abomination they would try to study and understand it, and when they failed to do so and inevitably went mad there would be no more arcane school. Inconceivable powers beyond time and space with inconsistent or otherwise impossible alien geometries which can bend reality itself and are sometimes inexplicably shaped like human internal organs for literally no reason whatsoever, must be kept far away from Arcane mages. As far away as possible.
