Magic leaves its mark. No magic is good or bad, only the intention behind it. But magic has an order, a balance, and when that balance is abused, it can create something equally skewed.
These formless, changeable things hunt magic – it made them, it is the life of these creatures. Commonly they’re too weak to do much harm, but grown ones will hunt down mages and sap them of their strength, or even gobble spirits whole. Terrible ones can do much worse. Magic cannot hurt them, nor can common blades, so fighting them can be dangerous work.
If you see these ghastly things yourself, run.