[An eerie smile spreads across Riku's lips, and he speaks in a low, spooky voice:]

To steal the life of its target, it slips into the prey’s shadow and silently waits for an opportunity.

[...]

Haunter is a dangerous Pokémon. If one beckons you while floating in darkness, you must never approach it. This Pokémon will try to lick you with its tongue and steal your life away. Its tongue is made of gas. If licked, its victim starts shaking constantly until death eventually comes.