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Transforms into the target, copying its stats (except HP), moves (at 5 PP each), type, weight, and ability. The stats are copied exactly, without regard to EVs, IVs, and level. IVs of the target are copied for purposes of Hidden Power, as well. Transform copies the target's species, which means that DeapSeaScale, DeepSeaTooth, Light Ball, Metal Powder, Quick Powder, Soul Dew, and Thick Club activate only if the target is the correct species. Level is not copied, so a low level Pokemon does less damage than a high level Pokemon when using the same move (because the attacker's level is part of the damage formula).
Choice Band, Choice Scarf, and Choice Specs keep their boost upon Transforming. The Transforming Pokemon is allowed to select a new move.
If a Pokemon Transforms into Giratina-O, it will instantly revert to the altered forme of Giratina, as it is not holding the Griseous Orb item.