Haunter does not have a set level at which it evolves. Rather, it evolves when it is traded to another player's game. Upon receiving the Pokémon, the recipient is notified of its evolution.
Throughout its evolution line, this Pokémon keeps its ghost and poison type. In addition to being immune to normal type attacks, Haunter and the rest of its evolution line gain immunity to damage-dealing ground-type moves through use of its ability "Levitate." The ability also grants this Pokémon immunity to spike traps and the "arena trap" ability. Additionally, it's unique types of ghost and poison grant it resistance to a wide variety of attacks. Attacks that are bug, grass, fairy or poison deal significantly less damage.