They spawn a lot as the second wave when a mini-anchor has undead as the first wave. Mini-anchors seem to always spawn in the same spot but I don't know how long the respawn timer is. Wiping out a few them might help you.
I mostly see them when I'm fighting worm cultists. Try looking for one of their camps. I know there's one in southern Malabal Tor that has some atronachs wandering around.
"He who fights with monsters should look to it that he himself does not become a monster...when you gaze long into the abyss the abyss also gazes into you..."