Prototype Portal + Ugin's Nexus + Lithatog

Prototype Portal

Ugin's Nexus

Lithatog

How it works

1. Activate Prototype Portal by paying 5 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.

By using Prototype Portal to repeatedly mint token copies of Ugin's Nexus, you set up an engine that bypasses the legend rule while weaponizing the Nexus's sacrifice clause. Whenever you feed one of these tokens to Lithatog's sacrifice ability, Ugin's Nexus replaces its trip to the graveyard by exiling itself instead—triggering its reward of an extra turn. Prototype Portal keeps the cycle going by churning out a fresh copy each rotation for just five mana, locking down the game through an endless sequence of extra turns.