Halee II was a barely livable planet with a hostile atmosphere and dangerously radioactive crystalline deposits. It was here that many Klingon outlaws had been banished from the empire.
By 2371 the Duras faction had built a secret spaceport with the help of some of these criminals which then acted as the base of their covert operations.
In 2371, Soran came to the secret base to synthesize trilithium from the unusual crystalline deposits native to the planet. Worf went undercover to foil his plans. (TNG video game: Generations)