Breathkeeper Seraph — Crimson Vow Commander #31

Flying, soulbond (You may pair this creature with another unpaired creature when either enters. They remain paired for as long as you control both of them.)

As long as Breathkeeper Seraph is paired with another creature, each of those creatures has "When this creature dies, you may return it to the battlefield under its owner's control at the beginning of your next upkeep."