Soulbond

Soulbond is a keyword mechanic introduced in Avacyn Restored. Soulbond pairs up creatures with each other. When a creature with Soulbond enters the battlefield, it can choose another unpaired creature. The creatures are considered paired with each other. The player may also choose not to pair with another creature, and then choose to pair with another creature when that creature enters the battlefield. Usually, the creature with Soulbond as well as its pair gain abilities or advantages when paired up. There are also other cards which look for paired up creatures.

Creatures can not become members of multiple pairs. However, if two creatures with Soulbond pair up, they each provide their benefits to each other. Thematically, the mechanic has similarities with the overly complicated banding.

Rules
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Rulings

 * Two paired creatures are still individual creatures in every way: they attack and block individually, they are targeted and affected by spells or abilities individually, and they change zones individually. If two paired creatures are attacking, blocking one of them has no effect on the other, for example.
 * Neither soulbond ability targets any creature.
 * You must control another unpaired creature at the moment a creature with soulbond enters the battlefield or the soulbond ability won't trigger at all. However, the creature that pairs with the creature with soulbond isn't chosen until the soulbond ability resolves.
 * If the pair is broken, the bonuses and abilities granted to the creatures immediately disappear. If the bonus included an increase to toughness, this may cause a creature to have damage marked on it equal to or greater than its toughness. If that happens, the creature is destroyed.
 * If becoming unpaired causes a creature to no longer have an activated ability, instances of that ability that have already been activated and are on the stack are unaffected.
 * Certain keywords granted to paired creatures are only relevant at a specific time, usually during combat, so becoming unpaired and no longer having that ability may have no effect on the current turn. For example, if a creature with reach blocks a flying creature and then loses reach, the creature with flying will still be blocked.
 * A creature with soulbond may grant an ability to itself and the creature it's paired with that includes the text "this creature." In such abilities, "this creature" refers only to the creature that has that ability, not the creature it's paired with.
 * If a creature with soulbond is paired with another creature with soulbond, each of them will receive both bonuses.
 * If you control multiple unpaired creatures with soulbond and another creature enters the battlefield, each soulbond ability will trigger. Soulbond abilities that try to resolve after you pair the creature will have no effect.
 * If a paired creature with soulbond loses soulbond, it and the creature it's paired with remain paired.