From Bulbapedia, the community-driven Pokémon encyclopedia.
The ??? type, unlike the Bird type, is an non-Glitch element type in existence from Generation I onward. However, it has never been applied to Pokémon, only to particular moves. When a bad egg occurs, it will be this type.
Introduced in Pokémon Gold and Silver, Curse is an attack with the ??? type.
Curse is an attack that behaves differently depending on the element type of the Pokémon using it, more specifically, if the Pokémon is of the Ghost type.
Shadow Rush is another Pokémon attack with the type ???.
In Pokémon Colosseum, Pokémon that have not been purified have the move Shadow Rush.
The ??? type runs off of the Pokémon's attack stat.