Yarus, Roar of the Old Gods + Karlov Watchdog + Harmless Offering + Phantom Train

Yarus, Roar of the Old Gods

Karlov Watchdog

Harmless Offering

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 loop relies on turning a neat timing restriction into a permanent-recycling engine, starting with Harmless Offering to hand your opponent Karlov Watchdog so its static ability forbids permanents from being turned face up *only* on your turn. Because Karlov Watchdog's restriction graciously lifts on your opponent's turn, you can use Phantom Train to sacrifice a face-down creature they won't let stay face up anyway, converting that sacrifice into a +1/+1 counter on the train. Each time that creature hits the graveyard, Yarus, Roar of the Old Gods triggers its replacement-style resurrection to return it face down, but because Karlov Watchdog suppresses the mandatory "turn it face up" clause during your turn, the creature safely resets right back into the graveyard to fuel Phantom Train's sacrifice cost all over again.