Culex is an optional boss who resides in Monstro Town in Super Mario RPG: Legend of the Seven Stars.
Unlocking Culex[]
The door to Culex is located in Monstro Town, and is initially locked. To unlock it, Mario must go to Moleville and purchase a set of fireworks from a mole child for 500 coins. Mario then exchanges the fireworks for a "shiny stone" with another mole child. Mario may return to Monstro Town and attempt to open the door again, which the "shiny stone" will unlock. Inside the residence, you will find yourself in a purple vortex. Culex will appear and challenge you to a battle.
Mario has the option to accept or decline this challenge at any time after opening the door, but lower-level players are cautioned against fighting as Culex is considered to be the toughest foe in the game.
The Battle[]
Culex alone has 4,500 HP, but he has four elemental crystals to aid him in battle, one each for earth, fire, wind, and water. Combined, this 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 elemental counterparts, but weak to their elemental opposites. Wind crystal is immune to thunder, but weak to jump. Earth crystal is the exact opposite; weak to thunder (Mallow's thunderbolt or shocker attacks), immune to jump. 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 defeat 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."