Helm of Obedience, and Dauthi Voidwalker

Prerequisites

• Helm of Obedience on the battlefield
• Dauthi Voidwalker on the battlefield
• available

Steps

1. Activate Helm of Obedience by paying and tapping it, causing target opponent to mill a card
2. Due to Dauthi Voidwalker's replacement effect, the milled card is exiled instead of going to the graveyard, and that opponents repeats that process until their library is empty

Results

• Exile target opponent's library