Meld cards are similar to double-faced cards and were introduced with Eldritch Moon. 
Each meld pair consists of two specific cards whose back faces combine to form one oversized Magic card face: Midnight Scavengers and Graf Rats meld to form Chittering Host; Hanweir Garrison and Hanweir Battlements meld to form Hanweir, the Writhing Township; and Bruna, the Fading Light and Gisela, the Broken Blade meld to form Brisela, Voice of Nightmares.
A meld card isn’t a double-faced card. It can’t transform or enter the battlefield transformed.
From the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- 712. Meld Cards
- 712.1. A meld card has a Magic card face on one side and half of an oversized Magic card face on the other. It doesn’t have a Magic card back.
- 712.1a There are three specific meld pairs. Each meld pair consists of two specific cards whose back faces combine to form one oversized Magic card face: Midnight Scavengers and Graf Rats meld to form Chittering Host; Hanweir Garrison and Hanweir Battlements meld to form Hanweir, the Writhing Township; and Bruna, the Fading Light and Gisela, the Broken Blade meld to form Brisela, Voice of Nightmares.
- 712.1b A meld card isn’t a double-faced card. It can’t transform or enter the battlefield transformed. (See rule 711, “Double-Faced Cards.”)
- 712.2. One card in each meld pair has an ability that exiles both that object and its counterpart and melds them. To meld the two cards in a meld pair, put them onto the battlefield with their back faces up and combined (see rule 701.35, “Meld”). The resulting permanent is a single object represented by two cards.
- 712.3. The front face of each meld card and the combined face formed by a meld pair each has its own set of characteristics.
- 712.3a While a meld card is outside the game, in a zone other than the battlefield, or on the battlefield with its front face up, it has only the characteristics of its front face.
- 712.3b While the two cards of a meld pair are on the battlefield as a melded permanent, the object represented by those cards has only the characteristics of the combined back face, and its converted mana cost is the sum of the converted mana costs of its front faces. If a permanent is copying a melded permanent, the converted mana cost of the copy is 0. See rule 202.3c.
- 712.3c Anything that needs information about a meld card or a melded permanent sees only the information given by the face that is currently up.
Example: A Clone enters the battlefield as a copy of Chittering Host (the combined back face of a meld pair). It has the characteristics of Chittering Host, even though the object it becomes is represented by only a single card, and its convered mana cost is 0.
- 712.4. If a melded permanent leaves the battlefield, one permanent leaves the battlefield and two cards are put into the appropriate zone.
Example: Chittering Host, a melded permanent, dies. An ability that triggers “whenever a creature dies” triggers once. An ability that triggers “whenever a card is put into a graveyard from anywhere” triggers twice.
- 712.4a If a melded permanent is put into its owner’s graveyard or library, that player may arrange the two cards in any order. If it’s put into its owner’s library, that player doesn’t reveal the order.
- 712.4b If a player exiles a melded permanent, that player determines the relative timestamp order of the two cards at that time. This is an exception to the procedure described in rule 613.6j.
Example: Duplicant is a card with the abilities “When Duplicant enters the battlefield, you may exile target nontoken creature” and “As long as a card exiled with Duplicant is a creature card, Duplicant has the power, toughness, and creature types of the last creature card exiled with Duplicant. It’s still a Shapeshifter.” As Duplicant’s first ability exiles Chittering Host, a melded permanent, Duplicant’s controller chooses whether the last creature card exiled is Midnight Scavengers or Graf Rats.
- 712.4c If an effect can find the new object that a melded permanent becomes as it leaves the battlefield, it finds both cards. (See rule 400.7.) If that effect causes actions to be taken upon those cards, the same actions are taken upon each of them.
Example: Otherworldly Journey is an instant that reads “Exile target creature. At the beginning of the next end step, return that card to the battlefield under its owner’s control with a +1/+1 counter on it.” A player casts Otherworldly Journey targeting Chittering Host, a melded permanent. Chittering Host is exiled. At the beginning of the next end step, Midnight Scavengers and Graf Rats are both returned to the battlefield, each with a +1/+1 counter on it.
Example: False Demise is an Aura with the ability “When enchanted creature dies, return that card to the battlefield under your control.” A Chittering Host enchanted by False Demise dies. The triggered ability returns both Midnight Scavengers and Graf Rats to the battlefield.
Example: Mimic Vat is an artifact that reads, in part, “Whenever a nontoken creature dies, you may exile that card.” A Chittering Host dies. As Mimic Vat’s triggered ability resolves, its controller makes a single choice and both cards that represented Chittering Host are either exiled or not.
- 712.4d If multiple replacement effects could be applied to the event of a melded permanent leaving the battlefield or being put into the new zone, applying one of those replacement effects to one of the two cards affects both cards. If the melded permanent is a commander, it may be exempt from this rule; see rule 903.9a.
Example: Leyline of the Void is an enchantment that reads, in part, “If a card would be put into an opponent’s graveyard from anywhere, exile it instead.” Wheel of Sun and Moon is an Aura with enchant player and the ability “If a card would be put into enchanted player’s graveyard from anywhere, instead that card is revealed and put on the bottom of its owner’s library.” If the controller of Chittering Host is affected by both cards’ effects, that player chooses one effect to apply to the event and Midnight Scavengers and Graf Rats are both moved to the appropriate zone.
- 712.4e If an effect needs to know the number of objects that changed zones, a melded permanent among those objects counts as one object that moved. If the effect needs to know the number of cards that changed zones, that melded permanent counts as two cards that moved.
- 712.5. Players who are allowed to look at a meld card may look at its half of the combined back face. Players may access Oracle text for the other member of the meld pair and the combined face at any time (see rule 108.1).
- 712.6. Meld cards in a player’s deck may be represented by checklist cards. See rule 713, “Checklist Cards.”
- 712.7. If a meld card is cast as a spell, it’s put on the stack with its front face up. See rule 601, “Casting Spells.”
- 712.8. A meld card enters the battlefield with its front face up unless it’s being melded with its counterpart.
- 712.9. If an effect allows a player to cast a meld card as a face-down spell, or if a meld card enters the battlefield face down, it will have the characteristics given to it by the rule or effect that caused it to be face down. That card remains hidden, using either a face-down checklist card or opaque sleeves. See rule 707, “Face-Down Spells and Permanents.”
- 712.10. Meld cards on the battlefield and melded permanents can’t be turned face down. If a spell or ability tries to turn such a permanent face down, nothing happens.
- 712.11. If an effect instructs a player to choose a card name, the player may choose the name of a front face of a meld card or the combined back face of a meld pair.
From the glossary of the Comprehensive Rules (Rivals of Ixalan (January 19, 2018))
- Meld Cards
- Cards with a Magic card face on one side and half of an oversized Magic card face on the other. See rule 712, “Meld Cards.”
- 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.
- However, draft events involving meld cards are identical as double-faced cards, which is revealed to all players until next pick (for non Pro REL events), or pre-sleeved before draft happened (for Pro REL events)
- 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.