Medomai the Ageless, Preeminent Captain, Arcane Adaptation, and Crystal Shard

Prerequisites

• Medomai the Ageless on the battlefield
• Preeminent Captain on the battlefield
• Arcane Adaptation on the battlefield
• Crystal Shard on the battlefield
• Medomai in hand
• Preeminent Captain does not have summoning sickness
• "Sphinx" chosen with Arcane Adaptation
• Opponents can not block and kill Preeminent Captain and Medomai
• each turn available

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 Crystal Shard by paying and tapping it, returning Medomai to your hand
5. Pass the turn
6. Repeat

Results

• Infinite turns