At his behest the Kami War began to regain That Which Was Taken.

O-Kagachi, Vengeful Kami

Flying, trample
Whenever O-Kagachi, Vengeful Kami deals combat damage to a player, if that player attacked you during their last turn, exile target nonland permanent that player controls.

Mystery Booster 2

Dominaria United Commander

Commander 2017

Flying, trample
Whenever O-Kagachi, Vengeful Kami deals combat damage to a player, if that player attacked you during his or her last turn, exile target nonland permanent that player controls.

Treasure Chest

A player attacked you if that player declared a creature as an attacking creature and that creature is attacking you. If that player attacked only planeswalkers you control, that player didn't attack you. If that player's creatures were attacking you only because they were put onto the battlefield attacking or only because you were reselected as the player they were attacking, that player didn't attack you.

If O-Kagachi deals lethal damage to a blocking creature and also deals damage to its controller, that blocking creature is destroyed before a target is selected for O-Kagachi's last ability. It can't be exiled.

O-Kagachi's last ability exiles only one nonland permanent, no matter how many creatures attacked you or how many times that player attacked you during their last turn.