Urza, Prince of Kroog + Timestream Navigator + Liquimetal Torque

Urza, Prince of Kroog

Timestream Navigator

Liquimetal Torque

How it works

1. Activate Liquimetal Torque's second ability by tapping it, causing Timestream Navigator to become an artifact until end of turn.
2. Activate Urza by paying 6, creating a token copy of Timestream Navigator that is a 1/1 Soldier creature.
3. Activate the oldest Timestream Navigator by paying 2, tapping it and putting it on the bottom of your library from the battlefield, causing you to take an extra turn after this one.
4. Repeat each turn.

This elegant loop relies on turning your extra-turn engine into an artifact so it can be endlessly replicated. With the city's blessing active, Timestream Navigator gives you the extra turn it needs by tucking itself away, but normally you'd lose the card forever. By using Liquimetal Torque to temporarily turn the Navigator into an artifact, Urza, Prince of Kroog steps in with his activated ability to forge a fresh token copy of it for six mana. Because token permanents can safely change zones before ceasing to exist, you can keep sacrificing and repeating the Navigator's effect every turn cycle while Urza's static lord ability pumps your growing army of 1/1 Soldier artifact creature tokens to boot.