Olivia, Crimson Bride, Medomai the Ageless, and Phyrexian Altar

Prerequisites

• Olivia, Crimson Bride on the battlefield
• Medomai the Ageless on the battlefield
• Phyrexian Altar on the battlefield
• Medomai in your graveyard
• Olivia does not have summoning sickness
• An opponent is unable to block your creatures

Steps

1. Declare Olivia as an attacker, attacking an opponent unable to block your creatures
2. Olivia triggers, returning Medomai from your graveyard to the battlefield tapped and attacking that opponent
3. Deal combat damage with Medomai, triggering its ability, causing you to take an extra turn after this one
4. Activate Phyrexian Altar by sacrificing Medomai, adding one mana of any color
5. Repeat

Results

• Infinite turns