The Star of Gobha was a mighty artifact crafted initially by Aulë, but shaped and altered by Sauron. Sauron under the guise of Shoglic presented the mighty artifact to Merro Glastanan, the first king of the Men of the White Mountains. Eventually it was placed in his crown and passed onto the King of the Dead who wore it until the days of Aragorn II's coming. The Star of Gobha had different powers based on the time periods of the Second Age and was counted as the most priceless artifact that the men of the White Mountains possessed.
Among the powers known during the time the King of the Dead held it in the Second Age were: Tripling the OB of the bearer, +7 PR stat bonus, double the morale and DB of the bearer, and the gem held a lot of spells the bearer could use.