Soulbond

Soulbound is a keyword mechanic introduced in Avacyn Restored. Soulbond pairs up creatures with each other. When a creature with Soulbound enters the battlefield, it can choose another unpaired creature. The creatures are considered paired with each other. If another creature enters the battlefield, the controller of the creature with Soulbound by choose to change the pair to the creature with Soulbound to the new creature. Usually, the creature with Soulbound 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 creatuers with Soulbound pair up, they each provide their benefits to each other. Thematically the mechanic has similarities with the overly complicated and obsolete Banding.

Reminder text

The latest reminder text for Soulbound reads You may pair this creature with another unpaired creature when either enters the battlefield. They remain paired for as long as you control both of them.