Invasion of Tarkir // Defiant Thundermaw

If a non-battle permanent that is already on the battlefield become a copy of a Siege, its controller chooses one of their opponents to be that battle’s protector. However, it will most likely be put into its owner’s graveyard because it has no defense counters (see below).

Damage dealt to a battle causes that many defense counters to be removed from it.

A battle’s “defense” is displayed in the bottom right corner of the card. A battle enters the battlefield with that number of defense counters. If another permanent enters the battlefield as a copy of a battle, it also enters with that number of defense counters.

If a battle that’s being attacked somehow stops being a battle, it is removed from combat. Similarly, if its controller changes in the middle of combat, it is removed from combat.

If a permanent that is represented by a transforming double-faced card becomes a copy of a Siege, it will be exiled as that Siege’s triggered ability resolves, then it will be cast transformed. Note that this applies only to transforming double-faced cards, not to modal double-faced cards that can normally be played using either face.

Sieges each have an intrinsic triggered ability. That ability is “When the last defense counter is removed from this permanent, exile it, then you may cast it transformed without paying its mana cost.”