Quandrix Command + Crystalline Crawler + Dualcaster Mage + Innkeeper's Talent

Quandrix Command

Crystalline Crawler

Dualcaster Mage

Innkeeper's Talent

How it works

1. Cast Quandrix Command by paying its mana cost, choosing its first and third modes.
2. Holding priority, cast Dualcaster Mage by paying its mana cost.
3. When Dualcaster Mage enters, it triggers, copying Quandrix Command, with the copy's first mode targeting Dualcaster Mage and the copy's third mode targeting Crystalline Crawler.
4. Resolve the copy of Quandrix Command, returning Dualcaster Mage from the battlefield to your hand and putting four +1/+1 counters on Crystalline Crawler due to Innkeeper's Talent.
5. Holding priority, activate Crystalline Crawler three times by removing three +1/+1 counters from it, adding 1.
6. Repeat from step 2.

This loop turns Quandrix Command and Dualcaster Mage into an infinite engine by continuously recasting the mage to copy the command. Crystalline Crawler acts as our mana and counter battery, fueling the sequence by converting the counters we place into the red mana needed to recast Dualcaster Mage. Meanwhile, Innkeeper's Talent supercharges the whole operation with its Level 3 ability, doubling the +1/+1 counters placed on Crystalline Crawler so we net extra mana on every single iteration.