Moritte of the Frost | Omarthis, Ghostfire Initiate | Mirror Box

Cards

Moritte of the Frost in hand or in the command zone.
Omarthis and
Mirror Box on the battlefield.
({2} magic symbol) ({G} magic symbol) ({U} magic symbol) ({U} magic symbol) plus enough mana to pay for commander tax, if applicable available.

Cast
Moritte of the Frost by paying ({2} magic symbol) ({G} magic symbol) ({U} magic symbol) ({U} magic symbol) plus commander tax (if applicable).
Choose to have
Moritte of the Frost enter the battlefield as a copy of
Omarthis with two +1/+1 counters on it.
Omarthis's first triggered ability triggers, putting a +1/+1 counter on itself.
Moritte of the Frost triggers, putting a +1/+1 counter on itself.
Repeat from step 3 any number of times.
End the loop by declining to put a +1/+1 counter on one of your creatures.
Warning:
Omarthis's last ability can increase the risk of losing by drawing from an empty library if you place enough counters on them.