
Jenova Cells — Whenever Sephiroth attacks, you may put a cell counter on target creature. Until end of turn, each modifiedmodifiedEquipment, Auras you control, and counters are modifications. creature you control has base power and toughness 7/5. (Equipment, Auras you control, and counters are modifications.)
The Reunion — , Sacrifice a modifiedmodifiedA creature is modified if it has a counter on it, is equipped, or is enchanted by an Aura you control. creature: Return this card from your graveyard to the battlefield tapped.
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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.