Cataclysm

Cataclysm is an X-cost Fire Epic that deals 7 damage times X with 3 Burn, exhausting after use. Dumping all your energy into this card creates massive burst turns. Energy generation from Purifying Flame, Void Exchange, Dark Ritual, and Foresight lets you fuel bigger X values.

Level 3 adds +3 damage per hit, and level 5 adds +2 Burn, scaling phenomenally with extra energy. The Fire synergy bonus damage applies on top, and Inferno Core doubles that bonus. Phoenix Feather gives +1 energy back on exhaust, which is less useful here since it fires after the card resolves.

Best paired with Inferno Goddess or Flame Aura for persistent Burn stacking. Enhanced versions get x1.2 to the damage per hit. One of the best finisher cards in the game for Fire-heavy builds.