Zirda, the Dawnwaker, and Grim Monolith

Prerequisites

• Zirda, the Dawnwaker on the battlefield
• Grim Monolith on the battlefield

Steps

1. Activate Grim Monolith's first ability by tapping it, adding
2. Activate Grim Monolith's second ability by paying , untapping it
3. Repeat

Results

• Infinite colorless mana