Historical Rules · 18 February 2022
Rule 702.95a
Official source text from the 18 February 2022 Rules release
Soulbond is a keyword that represents two triggered abilities. “Soulbond” means “When this creature enters the battlefield, 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 enters the battlefield under your control, 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.”