Phantom Steed, Time Walk, and Timeless Witness

Prerequisites

• Phantom Steed on the battlefield
• Time Walk in hand
• Timeless Witness in exile
• each turn available

Steps

1. Cast Time Walk by paying its mana cost, causing you to take an extra turn after this one Declare Phantom Steed as an attacker
2. It triggers, creating a token copy of Timeless Witness that's tapped and attacking
3. When the Timeless Witness token enters, it triggers, returning Time Walk from your graveyard to your hand
4. Repeat each turn

Results

• Infinite turns
• Lock