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Whenever Bone Dancer attacks and isn't blocked, you may put the top creature card of defending player's graveyard onto the battlefield under your control. If you do, Bone Dancer assigns no combat damage this turn.
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.