Prototype Portal + Ugin's Nexus + Krark-Clan Grunt

Prototype Portal

Ugin's Nexus

Krark-Clan Grunt

How it works

1. Activate Prototype Portal by paying 5 and tapping it, creating a token copy of Ugin's Nexus.
2. Activate Krark-Clan Grunt 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 Krark-Clan Grunt ability.
5. Repeat each turn.