Thalakos Deceiver

Oracle Text

Shadow (This creature can block or be blocked by only creatures with shadow.)

Whenever Thalakos Deceiver attacks and isn't blocked, you may sacrifice it. If you do, gain control of target creature. (This effect lasts indefinitely.)

Card Rulings

4/15/2013 An ability that triggers when something “attacks and isn’t blocked” triggers in the declare blockers step after blockers are declared if (1) that creature is attacking and (2) no creatures are declared to block it. It will trigger even if that creature was put onto the battlefield attacking rather than having been declared as an attacker in the declare attackers step.