Metalwork Colossus + The Ooze + Ravenous Intruder + Arwen, Weaver of Hope

Metalwork Colossus

The Ooze

Ravenous Intruder

Arwen, Weaver of Hope

How it works

1. Cast Metalwork Colossus by paying 0, which enters with at least two +1/+1 counters on it due to Arwen.
2. Activate Ravenous Intruder by sacrificing Metalwork Colossus.
3. When Metalwork Colossus dies, The Ooze triggers, creating a Mutagen token for each +1/+1 counter Metalwork Colossus had.
4. Resolve the Ravenous Intruder ability.
5. Activate Metalwork Colossus by sacrificing two Mutagens, returning it from your graveyard to your hand.
6. Repeat.

This engine uses Ravenous Intruder as a hungry outlet to sacrifice Metalwork Colossus, which is uniquely enabled by Metalwork Colossus's own cost reduction making it free to recast. As Metalwork Colossus enters the battlefield, Arwen, Weaver of Hope ensures it arrives with a fresh crop of +1/+1 counters. When the Colossus hits the graveyard, The Ooze watches it leave and generates a bounty of Mutagen tokens thanks to those counters. You can then use two of those newly minted Mutagens to pay the activation cost to return Metalwork Colossus straight back to your hand, fueling the loop infinitely.