Necrotic Ooze

The 4/3 body for four mana is incidental; the rules text is a combo engine wearing a creature costume. The borrowing is surgical: it copies only the activated abilities of creatures in graveyards, never their static or triggered ones, and it grants them on the Ooze itself, carrying the Ooze's own mana value and creature type. That precision is the entire machine. The classic line seeds two creatures into a graveyard whose activated abilities feed each other, then loops one ability into the other: a tap-for-mana cost paying for a draw cost, a "pay X, do Y" cost that something else refunds. Triskelion plus Phyrexian Devourer is the textbook pairing, but the deckbuilding stays open because the card reads from all creature cards in all graveyards, so an opponent's milled or dredged board can break the symmetry in your favor too. The grant is a continuous effect, not a one-time check on resolution, which is the wrinkle that makes the card forgiving: the Ooze does not need its combo pieces staged underneath before it lands. Mill a key creature into the graveyard after the Ooze is already in play and it gains that ability the instant the card hits the pile, no recast required. This is a graveyard-as-toolbox card rather than a graveyard-as-recursion card, paying you not for what you can return to play but for what you can borrow without ever lifting it out of the pile.