Because of its ability to slip through block walls, it is said to be from an other dimension.
By licking, it saps the victim's life. It causes shaking that won't stop until the victim's demise.
In total darkness, where nothing is visible, HAUNTER lurks, silently stalking its next victim.
Its tongue is made of gas. If licked, its victim starts shaking constantly until death even tually comes.
It hides in the dark, planning to take the life of the next living thing that wanders close by.
— Pokédex entries (via PokeAPI)