Leonin Relic-Warder

When this creature enters, you may exile target artifact or enchantment.
When this creature leaves the battlefield, return the exiled card to the battlefield under its owner's control.

The List

Commander 2017

When Leonin Relic-Warder enters the battlefield, you may exile target artifact or enchantment.
When Leonin Relic-Warder leaves the battlefield, return the exiled card to the battlefield under its owner's control.

Mirrodin Besieged

If Leonin Relic-Warder leaves the battlefield before its first ability has resolved, its second ability will trigger and when it resolves, do nothing. Then its first ability will resolve and exile the targeted artifact or enchantment with no way to return it to the battlefield.