Prototype Portal + Ugin's Nexus + Ravenous Intruder

Prototype Portal

Ugin's Nexus

Ravenous Intruder

How it works

1. Activate Prototype Portal by paying 5 and tapping it, creating a token copy of Ugin's Nexus.
2. Activate Ravenous Intruder by acrificing 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 Ravenous Intruder ability.
5. Repeat each turn.