Medomai the Ageless, Satoru Umezawa, and Crystal Shard

Prerequisites

• Medomai the Ageless in hand
• All other permanents on the battlefield
• You control an additional nontoken creature that does not have summoning sickness
• An opponent is unable to block creatures you control
• plus enough mana to cast the additional creature available each turn

Steps

1. Declare the additional creature as an attacker, attacking an opponent unable to block it
2. When the creature is unblocked, activate Medomai's ninjitsu ability by paying and returning the creature to your hand, putting Medomai onto the battlefield tapped and attacking
3. Deal combat damage with Medomai, triggering its ability, causing you to take an extra turn after this one
4. Cast the additional creature from your hand by paying its mana cost
5. Activate Crystal Shard by paying and tapping it, returning Medomai from the battlefield to your hand
6. Repeat each turn

Results

• Infinite turns