We first encounter the Overmage of the Cinbri at the end of Dungeon Siege II--The heroes defeat Valdis for the first time, then the "Azunite Scholar" who shadowed their progress in restoring the Aegis appears for the plot twist that sets up the cliffhanger ending to Dungeon Siege II: The Overmage reveals that he has manipulated both Valdis and the heroes into a second apocalyptic cataclysm for his own purposes. The world as it is known is destroyed.
The Overmage is the only member of the Cinbri race left in Aranna (for unclear reasons, he sacrificed even his brethren, the five Dark Wizards of Valdis, in his plot to bring about the cataclysmic end in Dungeon Siege II). He is well over one thousand years old, and is a master wizard and scholar.
The Overmage believes in order and control. He is driven by his desire to return his race to Aranna and present himself as supreme ruler. The Overmage will do anything to establish order and control, even if it means killing thousands of innocents in the process. He believes future generations will understand the rightness of his actions in the end, and thank him for having the foresight to take action.
The Overmage likes to work behind the scenes and is far too intelligent to get drawn into a fight he can't win. Instead of exerting force, he manipulates his foes by uncovering their deepest desires.
Fighting the Overmage of the Cinbri is actually a very boring affair as it is heavily scripted. He is invulnerable while summoning creatures, and continues to be invulnerable (though this time visibly so with a blue aura) while any of the summons survive. Once all summons are defeated, he appears weakened and collapses. During this time, he can be harmed.
However, this period of vulnerability is also scripted. While it seems to lend urgency for the heroes to attack him, he will actually not get up and summon another wave of monsters until a certain amount of damage is inflicted on him (you can try this by having your heroes stand down from their attacks and just loiter about). The battle with the Overmage of the Cinbri is, therefore, just a series of encounters with five strong miniboss creatures at a time. Considering that Broken World had introduced many interesting special effects for the earlier bosses in the game, the lack of new fantasy art to see was somewhat disappointing.