Omarthis, Ghostfire Initiate + Mirage Mirror + Vanquisher's Banner + Mirror Gallery

Omarthis, Ghostfire Initiate

Mirage Mirror

Vanquisher's Banner

Mirror Gallery

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.

By pairing Mirror Gallery to bypass the legend rule, Mirage Mirror can safely transform into a copy of Omarthis, Ghostfire Initiate without threatening to send either legend to the graveyard. Once this foundation is laid, triggering Omarthis's growth creates an infinite loop where Mirage Mirror and Omarthis continually feed +1/+1 counters onto each other back and forth. Vanquisher's Banner sits happily alongside them, leveraging the creature type shared by your growing threats to draw you cards as you cast spells, though you'll want to watch your library's size so that Omarthis's eventual death doesn't accidentally deck you when it manifests all those cards!