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

Basalt Monolith

Agatha of the Vile Cauldron

Karn, Silver Golem

How it works

1. Activate Basalt Monolith's first ability by tapping it, adding CCC.
2. Activate Karn by paying 1, 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 2, untapping it.
4. Activate Basalt Monolith's first ability by tapping it, adding CCC.
5. Repeat from step 3.