Yarus, Roar of the Old Gods, Karlov Watchdog, Shilgengar, Sire of Famine, and Switcheroo

Prerequisites

• Yarus, Roar of the Old Gods on the battlefield
• Karlov Watchdog on the battlefield
• Shilgengar, Sire of Famine on the battlefield
• Switcheroo in hand
• An opponent controls a creature
• You control a face-down creature whose front face is a permanent card
• You have enough mana to cast Switcheroo

Steps

1. Cast Switcheroo by paying its mana cost, giving control of Karlov Watchdog to an opponent
2. During the opponent's turn, activate Shilgengar 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 Shilgengar ability
5. Repeat from step 2

Results

• Infinite LTB
• Infinite ETB
• Infinite sacrifice triggers
• Infinite death triggers
• Infinite Blood tokens