Prototype Portal, Ugin's Nexus, and Lithatog

Prerequisites

• Prototype Portal on the battlefield
• Ugin's Nexus in exile
• Lithatog on the battlefield
• each turn available

Steps

1. Activate Prototype Portal by paying and tapping it, creating a token copy of Ugin's Nexus
2. Activate Lithatog by sacrificing the Ugin's Nexus token
3. The Ugin's Nexus token is exiled instead of being put into your graveyard, causing you to take an extra turn after this one
4. Resolve the Lithatog ability, giving it +1/+1 until end of turn
5. Repeat each turn

Results

• Infinite turns