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

Prototype Portal

Ugin's Nexus

Krark-Clan Ironworks

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 Ironworks by sacrificing the Ugin's Nexus token, adding CC.
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. Repeat each turn.

This clever setup turns Prototype Portal into a relentless engine for extra turns by exploiting the unique replacement effect of Ugin's Nexus. Because tokens go to the graveyard briefly before ceasing to exist, sacrificing the Ugin's Nexus token to Krark-Clan Ironworks triggers its graveyard-exile clause, successfully granting you an extra turn without ever breaking the game's rule against taking extra turns (since the first clause's restriction only applies to *beginning* an extra turn created by other means). Finally, Krark-Clan Ironworks conveniently refunds two generic mana toward the five needed to activate Prototype Portal again on the following turn, sealing the infinite loop.