Rule 359.3.f.4

Some information used by the effect of a triggered ability is referenced from the triggered ability itself, such as “enemy” and “friendly” status. This information is checked on execution of the referencing instruction. Example: Yasuo, Remorseful reads “when I attack, deal damage equal to my Might to an enemy unit here.” Yasuo moves to an occupied enemy battlefield and his attack trigger goes on the chain. In reaction to the attack trigger, the defending player plays a hidden Hostile Takeover and gains control of Yasuo. The triggered ability is unaffected by Yasuo changing controllers, and “enemy” is in reference to the triggered ability itself, so it will resolve with no issue. Example: In reaction to the same Yasuo, Remorseful trigger, say the defending player had instead played a spell that reads “[Reaction]. Gain control of a triggered ability. You may make new choices for it.” They chose the attack trigger. When Yasuo’s attack trigger resolves, if they didn’t make new choices for the trigger, the controller of the triggered ability will no longer be an enemy to the targeted unit, so the triggered ability will mistarget and do nothing. If they instead chose Yasuo with the attack trigger, he would be an enemy unit to the triggered ability and so it would deal damage equal to his Might to himself.