They do what those who dreamed them feared they would do.

But above killing, they priority feeding- by causing fear, or another emotion if another emotion fuels them (grief, anxiety, hatred, disgust, self-loathing), - or by consuming dream-matter, which they can harvest from natural sources of it once they are broken, or from the Forest Pools, Psychano, or Sea, among other places with loose, unformed dreamstuff.