The Ace of Nightmares has finally been found, just in time for the Darkmoon Faire to go live tomorrow!
The most elusive of the new Phase 3 Darkmoon Faire cards, the Ace of Nightmares is an integral part of the turn-in for the Darkmoon Card: Torment, a new trinket available as of Phase 3 of Season of Discovery. Up until now, no one had reported the location of this card's holder — but, on the eve of the Darkmoon Faire, Wowhead user Rfivealive555 reported that the card drops from Dreamscythe and Weaver in the new Sunken Temple raid!
The four new Darkmoon Decks debuting in Phase 3 provide several powerful trinkets after turning them in, including Darkmoon Card: Torment and Darkmoon Card: Sandstorm. With the discovery of the Ace of Nightmares, all the important Ace cards have now been found — so you can theoretically grab the trinket you want this Darkmoon Faire!
WoW Classic Darkmoon Faire Guide