Yarus, Roar of the Old Gods + Karlov Watchdog + Bazaar Trader + Phantom Train

Yarus, Roar of the Old Gods

Karlov Watchdog

Bazaar Trader

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 intricate loop hinges on an asymmetric control swap: by using Bazaar Trader to hand Karlov Watchdog over to an opponent, you weaponize its static ability to permanently suppress face-down cards. From there, you activate Phantom Train during that opponent's turn by sacrificing a face-down creature you control, sending it to the graveyard. That death triggers Yarus, Roar of the Old Gods, whose second ability catches the dying permanent and returns it to the battlefield face down. Normally Yarus would flip it right back up, but Karlov Watchdog's active curse stands in the way, keeping the creature stuck safely face down so you can sacrifice it all over again to Phantom Train for infinite death triggers, sacrifice triggers, and a growing mountain of +1/+1 counters.