Basalt Monolith, Karn, Silver Golem, and Agatha of the Vile Cauldron

Prerequisites

• Basalt Monolith on the battlefield
• Karn, Silver Golem on the battlefield
• Agatha of the Vile Cauldron on the battlefield
• Basalt Monolith does not have summoning sickness
• Agatha of the Vile Cauldron has power 1 or greater

Steps

1. Activate Basalt Monolith's first ability by tapping it, adding
2. Activate Karn by paying , causing Basalt Monolith to become a 3/3 artifact creature until end of turn
3. Activate Basalt Monolith's second ability by paying at most , untapping it
4. Activate Basalt Monolith's first ability by tapping it, adding
5. Repeat from step 3

Results

• Infinite colorless mana