
Alnath II is a planet located along the Federation - Klingon border in the Beta Quadrant. The planet was also once home to an ancient civilization.
In 2268, the Vulcan science vessel T'Pau visited the planet with an Andorian expedition. After visiting the planet, the Vulcan crew began acting very erratically, and eventually killed each other.
Shortly after, the crews of the USS Enterprise and the IKS Terror were similarly affected. Together they discovered it was linked to an ancient artifact on the planet. (TOS novel: The Klingon Gambit)