Ysgithyrwyn was one of the daelkyr lords of Xoriat that invaded Eberron during the Âge des Monstres 9,000 years ago. During the Guerre Daelkyr, it is presumed that he was banished back to Xoriat by the Gatekeeper Druids. However, Sinnoch, a dolgaunt agent of Ysgithyrwyn, was transported from Xoriat to Eberron by an unstable portal. Once on Eberron, Sinnoch hid in the Bois-Obscur ou Forêt de la Nuit, gathering Dolgrim troops, and trying to discover a way to bring his master Ysgithyrwyn over to Eberron as well.

The opportunity arose when a Karrnathi Artificier, Elidyr Brochann, stumbled across Sinnoch's cave. Sinnoch tricked Elidyr into believing the dolgaunt would help him build a device that would allow the Karrnathi army into taming Symbiotes. However, the device, called the Overmantle, was actually designed to create a portal to allow Ysgithyrwyn through.

Sinnoch used the Overmantle, and opened a portal for his master. However, his first attempt failed, due to Elidyr's brother, General Vaddon Brochann, and Elidyr's niece Lirra. Ysgithyrwyn, though, managed to reach through the portal and touch Elidyr, corrupting him and forcing him to carry on Sinnoch's work. Elidyr and Sinnoch fled, where they retooled the Overmantle to instead create an area where the beings from both worlds could exist on one. Ysgithyrwyn was close to escaping, but again was stopped by Lirra and her father. Sinnoch was killed, but Elidyr escaped, still holding the desire to free Ysgithyrwyn. Ysgithyrwyn, meanwhile, became interested in Lirra, telling her “I hope we get to play again someday.”