Riley, Parasite Unleashed

Menace, Vigilance, Ward {2}
Whenever this creature attacks, defending player may sacrifice a nontoken creature, then create X Blood Tokens where X is the sacrificed creature's mana value and remove this card from combat.
Sacrifice X Blood Tokens: Add X mana in any color combination to only cast instant and sorcery spells.
At the beginning of your end step, if you control 6 or less Blood Tokens, transform Riley.