Primeval Spawn

The gatekeeping clause is the whole design conversation. A five-color 10/10 with lifelink, vigilance, and trample is a fine top-end body, but the exile-if-not-cast rider quietly outlaws every route that makes big creatures cheap: no reanimation, no polymorph effect, no cheat-into-play trick. If it wasn't hard-cast (or something in the cast paid nothing), it never touches the battlefield. That restriction is doing more than pricing the payoff; it forces the card to be earned the honest way, which is what makes the death trigger fair. When it leaves, you exile the top ten and get to free-cast any spells among them totaling ten mana value or less, a burst of value scaled precisely to the creature's own cost. The elegance is in the symmetry: a cast-and-paid-for spell that refunds itself as a ten-mana-value cascade off the top of your deck, on the way out. The leaves-the-battlefield trigger is the wrinkle worth noting: it fires whether the creature dies, gets bounced, or is exiled by your own effects, so the payoff rewards recurring it rather than protecting it, but the cast-only clause slams the door on looping it back for free. You cast it and spend mana every time, and every time the library pays you back. It is a big dumb five-color beater wearing a very deliberate lock on the door behind it.