GhostDark
Ghost and Dark types: weaknesses, resistances and Pokémon
Ghost and Dark Pokémon take double damage from Fairy moves, resist Poison moves and are immune to Normal, Fighting and Psychic moves.
The balance is even: 1 weaknesses and 4 resistances or immunities.
Ghost moves are super effective against Psychic and Ghost and not very effective against Dark. Dark moves are super effective against Psychic and Ghost and not very effective against Fighting, Dark and Fairy.
To beat these Pokémon, your best pick is Fairy moves. Avoid Poison, Normal, Fighting and Psychic moves, which deal little or no damage.
There are 2 Pokémon with this combination, such as Sableye and Spiritomb.