Curse

Curse (Japanese: のろい Noroi) is a move that has a different effect for -type Pokémon. Before Generation V, it was a -type move, and the only move of that type.

Non Ghost-type
When Curse is used by a non--type Pokémon, the user's Speed Stat will drop and in turn raise their Attack Stat and Defense Stat.

Ghost-type
When Curse is used by a Ghost-type Pokémon, the user will lose half of their max HP and in turn will place a curse on the opponent. This curse will cause the afflicted Pokémon to lose 1/4 of their max HP at the end of every turn, unless switched out. When combined with Mean Look or Block, it can be a destructive combination. It is much like burn.

Trivia
With the exception of Hidden Power, Curse is the only move that has effects that vary depending on its user.