The Soul Dew (Japanese: こころのしずくSoul Dew) is a type of held item introduced in Generation III. In Generations III to VI, it boosts the Special Attack and Special Defense of Latios and Latias; from Generation VII onwards, it boosts the power of their Psychic- and Dragon-type moves instead.
In the movie, Annie and Oakley searched for the Soul Dew as well as the two current Legendary guardians of the city, Latios and Latias. If the Soul Dew is attached to the Defense Mechanism of Alto Mare, anyone using the device can drown the city in water, block all exits in the city, and awaken the Fossil Pokémon Kabutops and Aerodactyl in the city museum. However, doing this will also result in the Soul Dew using up all of its power and disappear. It is implied that this Soul Dew contains the soul of a deceased Latios.
During the climax of the movie, the original Soul Dew was destroyed, but a new one was created when Latios sacrificed his life to save Alto Mare from a massive tidal wave, using his own spirit to create a replacement for the destroyed gemstone.