Not many details have really been revealed. It is known that Varn Gossam's father stole the Books of Laroth along with the Heartstone from Laroth's palace in the underworld as the Once-Elven began their rebellion and escaped to the Shadowed Vale. When Haerlond Gossam accused the young Auric Ulvin and the children that traveled with him of stealing the Heartstone and was going to have them executed, Varn believed his pleas of innocence and tried to sneak him out. His brother's army found him before they escaped, but when they began their attack Auric reached for the only faint mana source he could find--which turned out to be the sun, so when he channelized it in it broke the through the barrier of mist that set the Shadowed Vale apart from Creation and had stopped sunlight from ever reaching there. Most of the Once-Elves were blinded, but Varn saw a vision of Lugus in the light and was rewarded for his devoting to truth in trying to save the innocent children by become Lugus's first priest. Varn, his wife, and others who had helped Auric escape fled the Vale and were eventually found by the Malakim. The Books of Laroth were among the items they took with them.
Varn's wife Talia, who was a witch, recognized Auric's latent power and may have taught him some magic while he was in prison. I suspect that she may have been the one in possession of the Books (Varn doesn't seem like the kind to study the arcane knowledge of an evil sorcerer), and even wonder whether she actually stole the heartstone to precipitate the whole affair. (Speaking of affairs, she had one with Baron Duin von Halfmorn, and gave him many of the "good guys" secrets which the werewolf funneled to the enemy, in Kael's D&D campaign at least.) Sometimes I suspect that Talia isn't one of the good guys, and might even be working for Laroth. She may have done all of this to get the Books out into the Creation where they could be used. I typically assumed that the books were left in some Empyrean monastery library for centuries, but since the time scale seems to be getting shorter than I'd like (what we know about the scenarios seems to imply that the Sidar exist when Auric is approaching ascension, and that Auric is still a fairly young man, but Auric was 15 years old when he and the books that would found the Sidar came into creation) I'm thinking it may have to be something more active than that. That's why I'm thinking Talia may be involved.
When a shade trades his soul for immortality he doesn't just consume it himself, he gives it to Laroth who uses it to help him conquer the Netherworld and try to become the new God of Death. The Sidar revere Arawn and try to follow him, but he isn't interested. Laroth, however, is. They don't know it, but they have been a great service to the dark sorcerer.
All that Kael has revealed about Basium's attempt to destroy the Sidar that Basium has a very "you're either for us or against us" attitude, implying they simply refused to aid him in his crusade against demons. Of course, the fact that they were (unknowingly) helping Laroth overthrow his old master Arawn could have been a part of it.
Basium is the Archangel of Life (or was, as he defied his god and chose mortality, and hates the new god of life), created while Arawn had mastery over both eh spheres of life and death. He was created at the same time as his twin sister Gyra, the Archangel of Death. They have rather different personalities. Gyra did not fall. While I believe that she too (like most of Arawn angels) want her god to be more active, she did not rebel to serve herself or Laroth. She only very rarely gets involved in creation, to lash out against the undead or to protect the Sidar. It is she who stopped her brother from destroying them.
