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If held by Latias or Latios, raises the holder's Special Attack and Special Defense stats by 50%. No effect if the holder is neither of these Pokemon.
Soul Dew is hands-down the best item that Latias and Latios could be using—it's just a pity that Soul Dew is currently banned in OU, as it tips the Eon twins from solidly OU to top contenders in Ubers—as it is essentially a free Calm Mind, or put another way, a Choice Specs that enables you to switch moves and switch into special attacks with ease. Latios certainly has no qualms about using it, and Latias will often be persuaded to swap her Leftovers for the mystical orb. If you are using the Lati twins in Ubers, you really need look no further than Soul Dew.
Dreamyard, after capturing wild Latias/Latios (BW2).