It could be said that Baal was the first, oldest and most powerful of the Demons but this is an over-simplification.
Baal was the first of the Ancient Dragons created by Xanadu the Dragon Father and was perhaps the first sentient mortal to exist on Alusia. It was not until the death of his life-partner Anu that Baal sought the darker paths of magic and discovered an ancient evil that predates our world.
Baal fell further into darkness and caused the first war, whereafter his kin imprisoned Baal and banished him from our world. This conflict heralded the end of the age of Dragons.
In Spring 802wk, Baal's joined-twin sons Apollyon/Abaddon almost freed Baal. His plan was thwarted at the last minute by the betrayal of his brothers Ahriman and Istu, and the return of the last surviving eldar dragons. Following their last meeting on Alusia it is believed that Xanadu took his children to another level of existence, a "Dragon Heaven".
It is believed that none of them will be seen in our world again.
There is also a plane of the same name.