For eons, Zyas and Ahlu have been bitter enemies. Often Ahlu stopping Zyas's plans for total destruction or chaos, and often Zyas preventing Ahlu from proper rest. Ahlu has always been good and dutiful torwards the light while Zyas has often been the villian and attuned to the dark. The two provide a balance amongst each other, which is how this idea was made, adding in that I'm a fan of the (now known as 'non-disney') starwars. Also in The Old Republic sith do frequently use purple sabers so it does work. X3

Art by the always wonderful and talented  Keewata