Thundering Mightmare

Soulbond is a keyword that represents two triggered abilities. “Soulbond” means “When this creature enters, if you control both this creature and another creature and both are unpaired, you may pair this creature with another unpaired creature you control for as long as both remain creatures on the battlefield under your control” and “Whenever another creature you control enters, if you control both that creature and this one and both are unpaired, you may pair that creature with this creature for as long as both remain creatures on the battlefield under your control.”

A creature becomes “paired” with another as the result of a soulbond ability. Abilities may refer to a paired creature, the creature another creature is paired with, or whether a creature is paired. An “unpaired” creature is one that is not paired.

When the soulbond ability resolves, if either object that would be paired is no longer a creature, no longer on the battlefield, or no longer under the control of the player who controls the soulbond ability, neither object becomes paired.

A creature can be paired with only one other creature.

A paired creature becomes unpaired if any of the following occur: another player gains control of it or the creature it’s paired with; it or the creature it’s paired with stops being a creature; or it or the creature it’s paired with leaves the battlefield.