Rings of Brighthearth, Forsaken Monument, and Grim Monolith

Prerequisites

• Rings of Brighthearth on the battlefield
• Forsaken Monument on the battlefield
• Grim Monolith on the battlefield
• available

Steps

1. Activate Grim Monolith's first ability by tapping it, adding
2. Forsaken Monument triggers, adding
3. Activate Grim Monolith's second ability by paying
4. Rings of Brighthearth triggers, causing you to pay to copy Grim Monolith's ability
5. Resolve the first Grim Monolith ability, untapping it
6. Activate Grim Monolith's first ability by tapping it, adding
7. Forsaken Monument triggers, adding
8. Resolve the second Grim Monolith ability from step 3, untapping it
9. Repeat

Results

• Infinite colorless mana