Voltaic Key + Forsaken Monument + Rings of Brighthearth + Arc Reactor

Voltaic Key

Forsaken Monument

Rings of Brighthearth

Arc Reactor

How it works

1. Activate Arc Reactor by tapping it, adding CCC.
2. Forsaken Monument triggers, adding C.
3. Activate Voltaic Key by paying 1 and tapping it, targeting itself.
4. Rings of Brighthearth triggers, causing you to pay 2 to copy Voltaic Key's ability, targeting Arc Reactor.
5. Resolve both Voltaic Key abilities, untapping it and Arc Reactor.
6. Repeat.