Omarthis, Ghostfire Initiate | Mirage Mirror | Mirror Box

Cards

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