A Soul Dew is a held item that gives a 50% boost to Special Attack and Special Defense when held by Latias or Latios. It has no effect when held by other Pokémon. It can be obtained as a tier 3 special drop. It is also a drop from wild Latias and Latios.
The Soul Dew Clause battle rule can prevent Latios and Latias from holding a Soul Dew.