On PC playing Traditional Standard Ranked. Opponent [secretly] cast an "INVISIBLE" Unstoppable Slasher. No card was ever shown as being cast by the opponent at that time. Opponent went on to attack with the Slasher and other VISIBLE creatures to win the game. Also, I couldn't declare Blockers vs the Slasher, because I didn't/couldn't see it... Just before the opponent won, during combat, I noticed the Unstoppable Slasher was face up peeking out of the side of MY Library!!!