Vadrok, Apex of Thunder

The apex legends each turned a mutate trigger into a reward, but this is the only one that reads that trigger as a spell engine rather than a stat bump or a defensive wrinkle. Every time it stacks onto a body, you may recast a noncreature card of mana value 3 or less from your graveyard for free, which reframes what the mechanic is buying. The others convert mutations into size or protection; this one converts them into a repeatable free cast, refueled by whatever noncreature cards keep hitting the pile as you feed more mutations onto the stack. A cheap burn spell, a piece of removal, a mana rock, a card-draw sorcery: any of them return on the trigger. Because the creature has no flash, the mutation and its trigger resolve at sorcery speed on an empty stack, so the value it wants to reclaim is proactive (a Lightning Bolt to the face, a Read the Bones) rather than reactive interaction that needs an opponent's spell to answer. Two constraints pay for the engine: the mutate cost itself, at four mana with a colored tax, and the mana-value-3-or-less cap, which points the loop at efficient spells instead of bombs. Flying and first strike keep the growing pile lethal while it grinds, so it threatens to end the game rather than merely accrue advantage. Less a beater than a recursion engine that happens to fly.