Vault 87: Forced Evolution
Chapter I is a Threaten effect with a leash instead of a timer: the borrowed creature stays yours for as long as the Saga survives, which for a Saga means exactly two of your draw steps before it sacrifices itself and the loaner walks home. That built-in return date is the design constraint, and the middle chapter is how you make the temporary body still count once it leaves. Chapter II hardens any creature you control (including the one you just stole), growing it and stamping it Mutant; the point is not to keep the creature but to convert its size into a number for the finale. Chapter III draws cards equal to the greatest power among your Mutants, so whatever you buffed in the second act sets the ceiling on the refill.
The sharp part is that Mutant is not a tribe you are joining; it is a category the card manufactures on the spot. Nothing needs to be a Mutant beforehand: Chapter II mints one, and the bigger the target you tag, the fatter the final draw. Steal an opponent's largest non-Mutant threat with I, enlarge and convert it with II, and even though that creature returns to its owner when the Saga completes, you have already banked its power as raw cards. The counterplay lives in the same three-turn clock: kill the Mutant before the third counter lands and the draw shrinks or vanishes.
