Horn of Gondor + Mana Echoes + Voltaic Key + Rings of Brighthearth

Horn of Gondor

Mana Echoes

Voltaic Key

Rings of Brighthearth

How it works

1. Activate Horn of Gondor by paying 3 and tapping it.
2. Rings of Brighthearth triggers. Choose not to pay.
3. Resolve the Horn of Gondor trigger, creating a number of 1/1 Human Soldier creature tokens equal to the number of Humans you control.
4. When the Humans enter the battlefield, Mana Echoes triggers that many times.
5. Resolve a Mana Echoes trigger, adding C for each Human and/or Soldier you control.
6. Repeat step 5 for each remaining trigger.
7. Activate Voltaic Key by paying 1 and tapping it, targeting itself.
8. Rings of Brighthearth triggers, causing you to pay 2 to copy Voltaic Key's ability, targeting Horn of Gondor.
9. Resolve both Voltaic Key abilities, untapping it and Horn of Gondor.
10. Repeat.