Phantom Steed, Ardent Elementalist, and Time Walk

Prerequisites

• Phantom Steed on the battlefield
• Ardent Elementalist in exile
• Time Walk in hand
• 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 Ardent Elementalist that's tapped and attacking
3. When the Ardent Elementalist token enters, it triggers, returning Time Walk from your graveyard to your hand
4. Repeat each turn

Results

• Infinite turns
• Lock