The history of the shadow of Tenebrae is an interesting one. Like all of the shadows of the golden circle, the shadow of Tenebrae is linked to Amber. Tenebrae is different than the other shadows in the golden circle as it is unable to hold life on it. Through the randomness that is shadow, Tenebrae has no air on it, or any beings that can live without it. Many resources that are rare on other shadows; gold, silver, oil, gems... were all very common in Tenebrae. Tenebrae was seen by outsiders as a seeming treasure trove of resources that were unavailable to those that lived on other shadows. The shadow went a long time without any people on it till an expedition from Dormaguez was able to travel into the shadow from Dormaguez and set up an outpost there.
The members of the expedition team where aided with technology from Holmgard and after just a few expeditions, they were able to establish a permanent settlement. The royalty of Tenebrae came about from the leaders of the expedition and soon Tenebrae was an independent shadow of the golden circle. Excepted to the court of Amber, Tenebrae supplied resources to the other shadows of the golden circle and in time grew in strength. Although it was growing in power, it went generally unnoticed by the royal family of Amber due to it's generally unpleasantness.
In time, only one member of the royal family found interest in the shadow. Terridus, son of Elmdor, was declared lord of the shadow by Obrion. When he arrived at the shadow he set to work on the shadow and helped it grow even more in power. To this day Terridus, much like the shadow of Tenebrae is largely ignored by the rest of the golden circle and the court of Amber, however, it's importance is growing with every passing year.