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The user enters an semi-invulnerable state for the turn the move is used. The next turn, the user exits that state and deals damage to the target. If the move is prevented from being used after the user has entered the semi-invulnerable state, then the entire move is canceled, but PP is still deducted. The user cannot make a move between turns. If the user holds the item Power Herb, the user will not enter the semi-invulnerable state and simply deal damage to the target that turn. The user can be hit while in the semi-invulnerable state if it was previously targeted by Mind Reader or Lock-On, or if the opponent has No Guard. This move cannot be blocked by Protect or Detect, and if either of these moves is used, Shadow Force nullifies its effects for the remainder of the turn, so the user of Protect / Detect can be hit be other moves in a double or triple battle.
Shadow Force, the signature move of the Giratina formes, is unfortunately not a great move on it. While it does have high Base Power and the ability to avoid any attack, the fact that the opponent can easily switch to a Normal-type like Blissey or a Steel-type like Ferrothorn to absorb the attack on the charge up turn generally makes Shadow Force a bad choice. However, Arceus also has access to the move and can make some use of it. Specifically, Swords Dance Arceus-Normal and Arceus-Ghost can use it in place of Shadow Claw to deal a larger amount of damage to opposing Ghost-types.
Shadow Claw is a much weaker alternative that takes only one turn to execute.