Smaug the Impenetrable + Metalwork Colossus + Goblin Bombardment + Xorn

Smaug the Impenetrable

Metalwork Colossus

Goblin Bombardment

Xorn

How it works

1. Activate Goblin Bombardment by sacrificing Metalwork Colossus, dealing 1 damage to Smaug.
2. Smaug triggers, creating two artifact tokens due to Xorn.
3. Activate Metalwork Colossus by sacrificing the two artifact tokens, returning Metalwork Colossus from your graveyard to your hand.
4. Cast Metalwork Colossus by paying 0.
5. Repeat.

This explosive loop centers on Goblin Bombardment turning its own sacrifice outlet into a self-sustaining engine by pinging Smaug the Impenetrable for 1 damage. Because Smaug is indestructible, he survives this abuse while his triggered ability generates two Treasure tokens, an output supercharged directly by Xorn's replacement effect. You then use those very same Treasures to pay the activation cost to recur Metalwork Colossus from your graveyard to your hand. Finally, because Metalwork Colossus checks the total mana value of your noncreature artifacts—specifically the heavy investment of those Treasures—its cost is reduced to zero, allowing you to recast it for free and feed the cycle all over again.