Noah and Mio, Interlinked

Vigilance, hexproof, reach

Noah and Mio's power and toughness are equal to the number of fusion counters on it

Put X fusion counters on Noah and Mio where X is the mana spent to cast this spell. Whenever you cast a non-creature spell, remove a fusion counter. When you remove the last, put a number of +1/+1 counters equal to the number of fusion counters removed, then transform this spell. (It transforms into Ouroboros, Mysterious Vestige, a legendary 7/7 Alien creature).