Yarus, Roar of the Old Gods + Karlov Watchdog + Phyrexian Altar + Harmless Offering

Yarus, Roar of the Old Gods

Karlov Watchdog

Phyrexian Altar

Harmless Offering

How it works

1. Cast Harmless Offering by paying its mana cost, giving control of Karlov Watchdog to an opponent.
2. During the opponent's turn, activate Phyrexian Altar by sacrificing a face-down creature you control, adding one mana of any color.
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. Repeat from step 2.