Omarthis, Ghostfire Initiate + Mirage Mirror + Mirror Box

Omarthis, Ghostfire Initiate

Mirage Mirror

Mirror Box

How it works

1. Put a +1/+1 counter on a colorless creature you control.
2. Omarthis's first triggered ability triggers.
3. In response, activate Mirage Mirror by paying 2, causing it to become a copy of Omarthis until end of turn.
4. Resolve the Omarthis trigger, putting a +1/+1 counter on itself.
5. Mirage Mirror triggers, putting a +1/+1 counter on itself.
6. Omarthis triggers, putting a +1/+1 counter on itself.
7. Repeat from step 5 any number of times.
8. End the loop by declining to place a +1/+1 counter on a creature.
9. Warning: Omarthis's last ability can increase the risk of losing by drawing from an empty library if you place enough counters on them.

This clever loop relies on a clever bit of identity-copying to turn a single counter into an endless cascade of growth. When you place a +1/+1 counter on a colorless creature, Omarthis, Ghostfire Initiate triggers its ability to reward you with a counter of its own, which you intercept by using Mirage Mirror to dynamically transform into a copy of Omarthis on the stack. Because Mirror Box safely neutralizes the legend rule and lets you ignore any concerns about uniqueness, the newly minted Mirage-Omarthis triggers off its own copying process and loops endlessly with Omarthis, Ghostfire Initiate to rack up infinite +1/+1 counters.