Meld

Meld is a keyword action introduced in Eldritch Moon and means to turn two meld cards on the back side into one oversized card, if you control the proper pair.

Description
The design of Meld was inspired by the Un-card B.F.M. (Big Furry Monster) and the unreleased mechanic Link.

Two cards that meld change into one oversized card on the battlefield. That single card only exists on the backs of the two other cards. A melded card is a single permanent represented by two cards. If it is one creature, it behaves like any other creature does. It can attack and block. It has abilities that work. It can be enchanted or equipped. You can put counters on it. And so on. You can think of it as one big card while on the battlefield. A single spell that says "Destroy target creature" takes out the whole permanent.

One of the two cards has the actual keyword action in its rules text, the other card only has the reminder text: Melds with (card name).

Whenever the melded card leaves the battlefield, both cards go, and they each turn front face up again.

Cards with meld
There are three cards with meld in Eldritch Moon.
 * The common Graf Rats melds with the common Midnight Scavengers into the common Chittering Host
 * The rare Hanweir Battlements melds with the rare Hanweir Garrison into the rare Hanweir, the Writhing Township
 * The mythic Gisela, the Broken Blade melds with the rare Bruna, the Fading Light</c> into the mythic Brisela, Voice of Nightmares</c>

Rulings

 * Although meld cards look similar to double-faced cards, they are not considered to be double-faced cards. They don't transform—instead, their back faces combine in pairs to form a single oversized card face.
 * One card in each pair of meld cards has an ability that instructs you to exile the two cards and meld them. If you control more than one object with one of those names, you select one object with that name to exile.
 * When two cards are exiled and melded, they each leave the battlefield, then return together as one new untapped object with no relation to either of the objects that left the battlefield. Counters, Auras, Equipment, and other effects that affected those two cards don't affect the melded permanent.
 * Only two cards belonging to the same meld pair can be melded. Tokens, cards that aren't meld cards, or meld cards that don't form a meld pair can't be melded. If an effect instructs a player to meld cards that can't be melded, those cards remain in exile.
 * While a meld card is anywhere other than the battlefield, or is on the battlefield with its front face up, it has only the characteristics of its front face.
 * While a melded permanent is on the battlefield, it has only the characteristics of its combined back face. Any effects that modify how the new object enters the battlefield will consider only the combined back face.
 * The converted mana cost of a melded permanent is the sum of the converted mana costs of its front faces. A creature that becomes a copy of a melded permanent has only the characteristics of that combined back face, and its converted mana cost is 0.
 * A player prompted to name a card may name the combined back face, and each player has the right to know that combined back face's characteristics at all times.
 * If an effect moves a melded permanent to a new zone and then affects "that card," it affects both cards.

Trivia

 * The meld mechanic is likely based upon themes of the Body Horror genre, which involve gruesome mutations occurring to the body whether by biological, conventional, or supernatural threats, such as The Thing, Hellraiser, the Tetsuo film series, and various Cronenberg films.
 * In the artwork of some cards with the Meld mechanic, Emrakul's tentacles can be seen in the background before their meld form takes place, possibly foreshadowing Emrakul corrupting those in her path.