Grim Monolith, The Blackstaff of Waterdeep, and Zirda, the Dawnwaker

Prerequisites

• Grim Monolith on the battlefield
• The Blackstaff of Waterdeep on the battlefield
• Zirda, the Dawnwaker on the battlefield
• Grim Monolith does not have summoning sickness
• available

Steps

1. Activate Grim Monolith's first ability by tapping it, adding
2. Activate The Blackstaff of Waterdeep by paying and tapping it, causing Grim Monolith to become an artifact creature until end of turn
3. Activate Grim Monolith's second ability by paying , untapping it
4. Activate Grim Monolith's first ability by tapping it, adding
5. Repeat from step 3

Results

• Infinite colorless mana