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Deals damage on the turn after it is used and boosts the user's Defense by one stage on the turn that it is selected. If the move is prevented from being used after the user has charged up, then the entire move is canceled, but PP is still deducted. If the user holds the item Power Herb, the move will not require a turn to charge.
Like all moves that take a turn to charge up, Skull Bash is not a good competitive move. Unlike other charge-up moves like Shadow Force, Skull Bash does not allow the user to avoid attacks, but it does provide a temporary boost to Defense. Like all other charge-up moves, during the charge-up turn, the opponent can easily switch to a counter who can easily absorb the Skull Bash like Ferrothorn, giving the opponent momentum.
Return is stronger but does not boost Defense.
Double-Edge is much stronger, but does not boost Defense and has recoil damage.