I'd like to see the damage per creature reduced to 1, the per-turn cap removed, the cost reduced, and allow them to stack, but maybe that's just me. Rage Potion, Firestorm (!!), synergy with Voodoo Doll, etc. Since Firestorm requires enemy creatures, I don't think it'd be OP at all.
Just a suggestion, but with the concept and the fact that it's a permanent, maybe call it something like "Furnace"? When I saw "Inferno," I expected to see a spell.