RockGhost
Rock/Ghost weaknesses
Rock/Ghost is weak to Water, Grass, Ground, Ghost, Dark and Steel.
When Rock/Ghost Pokémon are hit
2×Super effective
½×Not very effective
¼×Barely scratches it
0×No effect
Its two types separately
Multiply the two to get the result. For example, Water deals 2× × 1× = 2×.
No Pokémon is Rock/Ghost yet
This combination hasn't appeared in any main game, including forms and Megas.