From Bulbapedia, the community-driven Pokémon encyclopedia.
Soul Dew (Japanese: こころのしずく Soul Droplet) is a held item obtainable in Pokémon Ruby, Sapphire, and Emerald and Battle Revolution. It is found held by a wild Latios or Latias (depending on version) that resides on Hoenn's Southern Island, and it may be purchased for 14,400 Poké Coupons from the Mystery Gift shop in Pokémon Battle Revolution once the player has completed Stargazer Colosseum. It raises the Special Attack and Special Defense of Latios or Latias by 50% when held. Because of this large boost, it has been banned from official tournaments. If used in the Battle Tower, Battle Hall or Battle Subway, Soul Dew's effect is nullified. Because Poké Transfer bans the transport of Pokémon with held items to Pokémon Black and White, Soul Dew is currently unobtainable in Generation V.
In the games
In Pokémon HeartGold and SoulSilver, if the player has received the Enigma Stone from a Nintendo Event, the player can take it to Steven Stone in the Pewter Museum of Science, who will have a researcher analyze it who finds out that it really is Soul Dew. After he confirms it, the player will go outside the Museum with Steven Stone and will be able to battle one of the Eon Pokémon, depending on the game version (Latios in HeartGold or Latias in SoulSilver).
In the anime
sight-sharing with Latios
while in Soul Dew form after its sacrifice
In the anime, it appeared in the movie Pokémon Heroes, in which it is used to guard Johto's Alto Mare. It was given to an old couple in Alto Mare long ago after Latios and Latias saved the city. The Team Rocket duo Annie and Oakley were looking for Soul Dew as well as the two legendary guardians of the city, Latios and Latias. If it is attached to the Defense Mechanism of Alto Mare (DMA), anyone using the device can drown the city in water, block all exits in the city and awaken the Fossil Pokémon, Kabutops and Aerodactyl. It is implied that this Soul Dew contains the soul of a deceased Latios.