If a Siege never had defense counters on it (perhaps because a permanent became a copy of one), it can’t have its last defense counter removed. It will be put into its owner’s graveyard. You won’t exile it or cast the other face.
Invasion of Ergamon // Truga Cliffcharger
As a Siege enters the battlefield, its controller chooses an opponent to be its protector.
If a battle has no defense counters, and it isn’t the source of a triggered ability that has triggered but not yet left the stack, that battle is put into its owner’s graveyard. This is a state-based action. This doesn’t cause a Siege’s intrinsic triggered ability to trigger.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Manuel Castañón
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
