Encountered on Morgaath during Shadow of the Black Claw, Rugarix, later known as Eclipse, was only ever heard of as the most intelligent of the Wyrm Lords. Eclipse aimed to use the carnage of this conflict in a ritual to summon Tiamatto the Material Plane.
Later during Ret's Rebellion, Eclipse was noted as guarding the ancient Wood Elven capital Emerald, cutting off access to the royal vaults.
During Kingmaker, Eclipse was noted to be back on Morgaath, working with Varanthian & raising a Dracolisk. The Council of Redemere would eventually learn that Eclipse had been working with Nyrissa, even establishing a lair in her realm The House at the End of Time. During the climax of the conflict with Nyrissa, Eclipse fought the dragons known as Nalesky & Domani. But, Eclipse's lair seemed to have disappeared from Nyrissa's realm before it was all over.
Legends of Eclipse have gone back for ages, including involvement with the fall of the Kingdom of Rhest, and facilitating Nyrissa's agenda with their collapse. They say that Eclipse was already in history before Ragnarok, but known as Paragon. As Paragon, it was known to the sole paladin of some evil god.