Yarus, Roar of the Old Gods + Karlov Watchdog + Zedruu the Greathearted + Phantom Train

Yarus, Roar of the Old Gods

Karlov Watchdog

Zedruu the Greathearted

Phantom Train

How it works

1. Donate Karlov Watchdog to an opponent.
2. During the opponent's turn, activate Phantom Train by sacrificing a face-down creature you control.
3. When the creature dies, Yarus's second ability triggers, returning it from your graveyard to the battlefield face-down as a 2/2 creature, and then not turning it face-up due to Karlov Watchdog.
4. Resolve the Phantom Train ability.
5. Repeat from step 2.

This clever loop relies on Zedruu the Greathearted's ability to generously gift away Karlov Watchdog to an opponent, locking down their permanents so they can't be turned face up on your turn. With that restriction in place, sacrificing a face-down creature to fuel Phantom Train's activation sends it to the graveyard, where Yarus, Roar of the Old Gods immediately swoops in to return it to the battlefield face down. Because Karlov Watchdog prevents the newly returned creature from turning face up, it stays safe and ready to be sacrificed all over again.