Litwick is a ghost-fire type Pokemon that debuted in the 5th Generation Pokemon Black & White games, and is #607 in the Pokedex. It is shaped like a candle with a face and a purple flame. It is the cutest dang soul-sucking Pokemon in the whole game, and my BFF's favorite Pokemon.
Oh, the soul-sucking? Well, Pokemon games have a history of hiding absolutely horrifying backstories in the Pokedex entries of otherwise adorable Pokemon, and Litwick is no exception. According to the assorted Pokedex entries, Litwick's flame is actually fueled by the souls and life-force of those around it.
The Pokedex Entries from Black and White respectively are:
"Litwick shines a light that absorbs the life energy of people and Pokémon, which becomes the fuel that it burns."
"While shining a light and pretending to be a guide, it leeches off the life force of any who follow it."
Now, Litwick is only actively sucking out life force when its flame is actually lit, but if you look in the games, the flame is always lit, meaning that not only is it sucking the life force from your other pokemon, but it's probably draining you as well. In Black/White 2, the direct sequel to the Black and White games, you are challenged by a trainer who has Litwick. When she loses, she tells you that "Losing makes [her] tired," implying that her Litwick is sucking out her life force to regain the health it lost.
At level 41, it evolves into Lampent. When Lampent is exposed to a special evolutionary stone called a Dusk Stone, it turns into Chandelure.