Dreameater  

Well,
Over the last 3 days I have cleared the room about 30 times, so that's roughly 120 placeholders for Dreameater. I have had at last count 12 pops of the evil cube, so that seems to be about a 1 in 10 chance from any placeholder or roughly once every 2-3 clears, which seems about right.

Unfortunately the RNG has not been kind to me and all 12 occasions have seen a ring drop and no augment for me. The chance of going 12 pops in a row without the augment if the aug has a 33% chance of dropping is pretty small, about 1%, but if the augment has only say a 10% chance of dropping per kill then 12 fails in a row is not that unlikely, about 30% in fact.

On the basis of this I'd affirm that Dreameater is a common spawn (about 40% chance of getting him at each clear of the room) but his augment is somewhat uncommon, probably 20% chance or less per kill.

By the way, why is a named CUBE (live mob, not undead) spawning from haunts, which are undead?