The Official Pokémon site has announced the release of another event for Pokémon HeartGold & Pokémon SoulSilver--the Enigma Crystal! Obtained via Wi-Fi from November 27th, 2009 through January 11th, 2010, this item will allow you to encounter either Latios (HeartGold) or Latias (SoulSilver), depending on your game version.

In order to encounter the Pokémon, you must take the Enigma Crystal to the man that revives fossils in the Pewter City Museum. He will identify it as the Soul Dew item. Steven Stone will speak to regarding the object. Soon after leaving, Latios or Latias will challenge you to a match.