Culex

Culex is an enemy in Super Mario RPG: Legend of the Seven Stars that lives in Monstro Town. He is also the final boss in the original Final Fantasy, and was put in Super Mario RPG to commemorate the team-up between Nintendo and Squaresoft (now Square-Enix).

Unlocking Culex
The door to Culex is located in Monstro Town, and is initially locked. To unlock it, one must go to Moleville and spend five hundred coins on a set of fireworks. The player then goes to a mole child and exchanges the fireworks for a "shiny stone." After that, go back to Monstro Town and attempt to open the door again. The shiny stone will unlock the door. Inside, you will find yourself in a purple storm. Culex will appear and challenge you.

The Battle
Culex alone has 8,000 hit points, but he has four crystals to aid him in battle (earth, fire, wind, and water). Combine all of them, and Culex makes for the toughest battle in the entire game, even more so than Smithy (the final Boss).

The fire and water crystals are immune to their elementary counterparts, but weak to their elementary opposites. Wind crystal is immune to jump, but weak to thunder (Mallow's thunderbolt or shocker attacks). Earth crystal is the exact opposite; weak to jump, immune to thunder. Culex has no weaknesses, and has a high magic defense (meaning he is harmed very little by magical attacks of any kind).

Reward
When you beat Culex, he will commend your spirit, and reward you with the Quartz Charm, which prevents instant death attacks, as well as increasing damage done by the wearer by fifty percent (it is not a flat buff, like most other items). This applies to both regular attacks, magic attacks, and even items that have a flat damage rate such as Rock Candy (instead of 200HP to all enemies, it does 300).

Culex will also break the fourth wall by saying "Perhaps, in another time, in another game, we were mortal enemies."