Prototype Portal + Ugin's Nexus + Grinding Station

Prototype Portal

Ugin's Nexus

Grinding Station

How it works

1. Activate Prototype Portal by paying 5 and tapping it, creating a token copy of Ugin's Nexus.
2. When the Ugin's Nexus token enters the battlefield, Grinding Station triggers, untapping Grinding Station.
3. Activate Grinding Station by tapping it and sacrificing the Ugin's Nexus token.
4. The Ugin's Nexus token is exiled instead of being put into your graveyard, causing you to take an extra turn after this one.
5. Resolve the Grinding Station ability, causing target player to mill three cards.
6. Repeat each turn.