
FlyingFlyingCan only be blocked by creatures with flying or reach., soulbondsoulbondYou may pair this creature with another unpaired creature when either enters. They remain paired for as long as you control both of them. (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 diesdiesA creature dies when it goes from the battlefield to a graveyard. Being exiled or returned to hand is not dying., you may return it to the battlefield under its owner's control at the beginning of your next upkeep."
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Read from the rules text. A card that enables a keyword makes or is the thing; a card that rewards it gets better when you have it. Two cards interact when one enables what the other rewards.