Battles are a card type introduced in March of the Machine. Damage dealt to a battle removes that many defense counters, and once the last one is gone, you exile it and may cast its transformed back face without paying its mana cost.
The appeal is value on both ends. Each battle does something the moment it enters, whether that's Invasion of Zendikar ramping two lands or Invasion of Ikoria tutoring a creature onto the battlefield.
