In the Pokmon video game series, Haunter is a Ghost/Poison-type Pokmon that evolves into Gengar. There are two methods to evolve Haunter into Gengar: trading it with another player, or leveling it up while it is holding a Reaper Cloth.
Trading Haunter with another player is the simplest method of evolution. Once the trade is complete, Haunter will immediately evolve into Gengar.