Giratina was banished from the pokemon world for violence. It silently gazes upon the old world from disortion world.
Litwick pretends to be a light to guide travelers and then it sucks their souls out
Duskull enjoys scaring children
Dusknoirs have the ability to receive radio waves from the spiritual world that tell it to bring people there. It has a mouth-like hole in its belly where it can hold lost spirits. Dusclops is like a black hole. If anyone tries to look at its beleives burning ball of fire it would suck them in and would never be seen again. It could hypnotize people to do its bidding. It looks for wandering will-o'-the-wisps and sucks them inside, it is a mystery what happens inside.
Chandelure sucks the spirits out of living creatures and then burns them. It leaves the body behind.
Lampent visit dying people and steal spirit from the body. They wander through cities to search for spirits of the fallen. Rarely, Lampent work alongside Litwick to lead people and Pokemon to the Ghost World while stealing their life energy.
Old stories say sableye will steal your soul if you look into its eye.
A Gengar might take delight in its victim's terror. They can cast curses.
Many people beleive Haunter is from another dimension. They hide in very dark places and plot to take the life of the next living thing that comes near.
Gastly may sneak up on unexpecting prey and put them to sleep or poison them through the skin.
Sometimes, when you hear a Misdreavous cry, you will be tormented by headaches and hallucinations.
Drifloon try to grab children and bring them to the underworld.
Cofagrigus is said to swallow those who get to close and turn them into mummies.
A Jellicent's favourite food is life energy. I think its pokemon black pokeex entry is a good summary. "The fate of the ships and crew that wander into Jellicent's habitat: all sunken, all lost, all vanished."
Frillish drags its opponents to the ocean floor.
Spritomb was formed by 108 spirits. As a punishment for its misdeeds, it was bound to a fissure in an odd key stone. Voices can be heard from it.
And they say pokemon is for kids. O.O
I didn't know there were that many freaky pokemon, there probably are more.