Added [Gold] to the title after realizing that the AI-abusive nature of this deck can really mess up an Arena deck. I can't count the number of times I've ended up using Nightmare on an opponent's creature in order to suck up several turns of their quanta and keep them from doing something important.
Example: getting dominated by an Arctic Squid, everything is frozen. Opponent empties his hand, and I Nightmare the Squid. Why? Because he's only gaining 4
per turn, and he keeps playing more and more Squids rather than using the one he has. I kill him before he can get back around do the freezing.
Similarly: Opponent is playing a Fire Spirit deck, and has crippled me by Exploding my Daggers and putting out a Fire Shield that is taking out my Photons instantly. He plays Fahrenheit, and I immediately Nightmare his Fire Spirits. He spends the next half-dozen turns playing Fire Spirits rather than pumping his existing Fire Spirits, so both his Fire Spirit damage and his Fahrenheit damage stay low. I finally play Stiletto-Animate-Blessing-Blessing-Blessing for a mob that will survive his Fire Shields for a while, and a couple of Crusaders to pick up the 15 point boost in damage (and healing), and it's an EM snatched from the jaws of almost certain defeat.
Nightmare is a god card against Arena (as zombie0 has already discovered with his Giant Killer). Day Traitors just makes it much, much nastier.