Embalm is an ability that works from your graveyard. If a card with embalm is in your graveyard, you can pay its embalm cost, exile that card, and create a token that's a copy of it. The token is white, has the creature type Zombie in addition to the card's other creature types, and has no mana cost. You can only do this during your main phase, when you could cast a sorcery, and only if the stack is empty. Once the card is exiled, you can't embalm it again.
In play, embalm turns a dead creature into a fresh one. It rewards decks that expect creatures to die, since removal or combat doesn't fully answer the card—you get a second body later. The token copies the original's power, toughness, and abilities, so even a small embalm cost can rebuild your board. Because the token is a Zombie, it can also fit tribal synergies.
