Be glad Tarquin is independent, if he was part of Team Evil, the OotS would be finished.
Xykon only succeeds through raw power from being an Epic Sorcerer, not through smarts.
Redcloak is smart, but nowhere near that clever or innovative, and doesn't have Xykon's raw power.
Tarquin is brilliant, but only wants to rule that continent from behind the scenes, not rule the world or work openly towards a larger goal. . .which is part of his brilliance, he doesn't over-reach.