I just tested Gruesome Slaughter with several colorless creatures (Breaker of Armies, Gold-Forged Sentinel). When cast, it gives this ability to any colorless creatures in play. If you play another colorless creature after Slaughter, for example, it won’t get the ability (okay, not so much a bug but just the way the card works in not exactly matching the card’s description). However, this buff doesn’t go away at the end of the turn. Basically, any colorless creatures that gets stuck with this ability will only attack enemy creatures for the rest of the game. If there is no enemy creature in play, it will just skip every one of its attacks… FOR THE REST OF THE GAME.
