Gravitic Punch + Kalamax, the Stormsire + Fork

Gravitic Punch

Kalamax, the Stormsire

Fork

How it works

1. Cast Gravitic Punch by paying its mana cost, targeting Kalamax.
2. Holding priority, cast Fork by paying its mana cost, targeting Gravitic Punch.
3. Kalamax's first and second abilities trigger.
4. Resolve the first ability, creating a copy of Fork targeting the original Fork.
5. Resolve the second ability, putting a +1/+1 counter on Kalamax.
6. Resolve the Fork copy, creating a copy of Fork, targeting the original Fork.
7. Kalamax's second ability triggers, putting a +1/+1 counter on itself.
8. Repeat from step 6 any number of times.
9. Once you have an arbitrarily large Kalamax, have the final Fork copy target Gravitic Punch. This'll create two copies of Gravitic Punch in addition to the original Gravitic Punch.
10. Resolve all Gravitic Punches, dealing infinite damage to each of up to three opponents.