"Ever see a wall drop dead of fright, kid? It ain't pretty."
—Cpl. Dobbs

Necrite

Whenever this creature attacks and isn't blocked, you may sacrifice it. If you do, destroy target creature defending player controls. It can't be regenerated.

Necrites killed Jherana Rure, ending the counterinsurgency.

Masters Edition II

Whenever Necrite attacks and isn't blocked, you may sacrifice it. If you do, destroy target creature defending player controls. It can't be regenerated.

Fifth Edition

Sacrifice Necrite: Bury target creature defending player controls. Use this ability only if Necrite is attacking and unblocked.

Necrites killed Jherana Rure, ending the counter-insurgency.

Fallen Empires

If Necrite attacks and is not blocked, you may sacrifice it to bury a target creature controlled by the player Necrite attacked this turn. If you do so, Necrite deals no damage during combat 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.