Tome of the Shade (Rift Lore)  

The fall of the Ethian Empire allowed Mathosia to truly prosper. The Kingdom grew until it expanded from Caer Mathos to Port Scion, and its influence spread over the civilized lands. Fearless and loyal to a fault the feudal Mathosians were pledged to serve the interests of their king over all else. When Sons Zareph and Aedraxis were born without a clear line of succession, the kingdom was thrown into a civil war.

The Shade War exposed how complacent the kingdom had grown, the dragon cults had corrupted many of the noble houses, especially those loyal to Prince Aedraxis who had taken up the worship of Regulos. The war ended when Aedraxis unleashed the shade energy upon the world and ushering in the second age of monsters.