Charnelhoard Wurm

The recursion looks like the headline, but the trample is what keeps the engine reliable. Plenty of large creatures have hung a graveyard refund off combat damage, and most of them shut off cleanly under a single chump blocker. Here a chump does not stop the trigger: trample shoves at least a point past the blocker, that point lands on an opponent, and the return fires anyway. The 6/6 eats the blocker and still pays you a card for the trouble. The wording widens the window too. The trigger reads "deals damage to an opponent" rather than "combat damage," so any damage this creature itself deals to a player counts, not just the attack step: a double-strike grant lets one connection trigger twice, and a spell like Soul's Fire or Chandra's Ignition that makes the Wurm deal its own damage to an opponent fires the trigger, because the Wurm is the source. What stays outside the trigger is damage the Wurm never deals: a burn spell aimed at a player is the spell's damage, not the Wurm's, so it does nothing here. The card returned is unrestricted (a bomb, a piece of removal, a land, whatever the yard holds), and the fact that it returns to hand rather than the battlefield is what keeps the loop fair: you pay the recast every time it connects. This sits in the lineage of expensive midrange beaters that earn back their investment, with the refund gated behind getting damage through rather than handed over on arrival. The answer is to deal with it before it connects, not after.