Medomai the Ageless, Preeminent Captain, Arcane Adaptation, and Erratic Portal

Prerequisites

• Medomai the Ageless on the battlefield
• Preeminent Captain on the battlefield
• Arcane Adaptation on the battlefield
• Erratic Portal on the battlefield
• "Samurai" chosen with Arcane Adaptation
• Oathkeeper attached to Filigree Familiar

Steps

1. Declare Preeminent Captain as an attacker
2. Preeminent Captain triggers, putting Medomai from your hand onto the battlefield tapped and attacking
3. Deal combat damage with Medomai, causing you to take an extra turn after this one
4. Activate Erratic Portal by paying and tapping it, returning Medomai to your hand
5. Pass the turn
6. Repeat

Results

• Infinite turns