Some thought.
Nightmare - Aaron:
Love the idea to use Dissipate as HP. I think its attack should not be psychic since it is a Force creature. Psychic attack make it very bad if the opponent is guarded by a Nonliving, since the attack is not ranged.
Nightmare Knight:
I love Knightmare as a name
I would definitely make this a Soldier to allow War Mages to have something cool if they want to pay the 8 SBP cost.
Nightmare Eater:
It is really weak imo. Even if you are really lucky with the yellow die, you will deal 5 dice every 2 attacks. This average 2.5 dice per attack, for a 14 mana creature with an upkeep of 2. Its attack either need to not break sleep or to be way more powerful against sleeping creatures. Also, its attack is total garbage against a Mage.
I would make the attack bar:
Quick, Melee, 0 red dice, yellow dice 4-11: Tainted 12: 2 Tainted Ethereal, Unavoidable, No Damage, +3 vs Sleeping.
That way it doesn't break sleep and it is useful against Mages except Necromancer, which is ok, since it would be unfair to have a creature that you can't defend against because none of your creatures can see it.