The Sidar are not Once Elves, but a group of humans who revered Arawn. Sandalphon was among the first to discover the Once Elf city of Barathrum within the Shadow Rift, and among the first to study in the library there. This library was not especially large, but was of immense value because its volumes were mostly masterpieces published posthumously; they had been written in the lands of the dead and stolen from Laroth's palace when the Once Elves escaped. Among these books was a tome written by Laroth himself, which was filled with instructions for various spells and rituals. Sandalphon made a copy of this book and took it home with him. It was from this copy that the Sidar discovered how to become Shades.
Many years (perhaps generations) latter, a Shade named Rathus Demora began to suspect that Sandalphon's copy was filled with mistakes. He gained admittance to Barathrum's great library in order to study the original book and make a better copy of it. While there he performed the Waning ritual properly, and managed to not only become invisible but also to disappear completely from the memory of those who had admitted him.
Rathus entered Barathrum at the same time as a druidess/witch named Talia, and perhaps also a sorceress called Gosea the Dwindling. Gosea stole the Once Elves' most prized possession, a fist-sized lump of jagged grey rock called the Heartstone, which they had stolen from Laroth and taken out of the Netherworld. (Kael implies that this is the heart of Anesidora, the second wife of Nemed and the mother of all mortal races. Whoever possesses her heart can summon her spirit, which manifests as a dark draconic shadow that causes enemies to fall into a deep despair.) The Once Elves don't seem to know about Gosea (perhaps she learned the Waning ritual while there too?), so they use Talia as a scapegoat. Waldrun convinced Haerlond to let him use the taghairm ritual on her, killing her and enslaving her soul so that she would lose the ability to lie to them (or perhaps the ability to say anything but what the Necromancer wishes).
When Gosea was fleeing the Shadow Rift, she stopped in the Illian village of Brigdarrow. At this time the young Auric Ulvin and his friends Dain and Kirien had gone missing, as they were caring for an injured dog they had found. The Thegn of Brigdarrow suspected Gosea and her guards might be involved, and so had their possessions searched. They took away the Heartstone for a time, but gave it back and let her to once the children had returned. Once she left, she hired 3 clans of goblin mercenaries to destroy the village and kill anyone who might have seen her with this relic. The children were away at the time, as the dog had led them to find the young elf who was its master. When they saw the town was destroyed, they decided to go with the elf, who had been looking for Barathrum. (They had never heard of it, but his map showed that it was the only settlement close enough for them to reach with so little supplies. Brigdarrow was far from the heart of the Illian empire, so the next several closest towns were inhabited by Ljosalfar elves instead of their fellow Illians.) They would have been lost in the Whispering Woods if it were not for Auric, who was able to navigate by mana instead of relying on senses that were confounded by the Whiddershins Curse. (Normally anyone the Once Elves allow to visit would be given a charm to protect them from this confusion.) When Waldrun attacks and captured them, Auric frees an enslaved Sluagh that was set upon them. Waldrun has the children imprisoned for short time. In the jail of Barathrum, Talia gives Auric his first formal magic lesson. Waldrun cannot convince Haerlond that the children are guilty of anything, so he lets them go. Auric visits the library, and learns much that will eventually lead him along the path to godhood. Waldrun convinced Dain to enter the portal to the Netherworld to visit his dead father, knowing that Once Elf law would never let anyone who has seen that world ever pass to the other side of the Shadow Rift again and return to the land of the living. (Dain's entry into the netherworld provides cover for Rathus Denmora, who at the same time slips through the portal unseen. This might be the main reason for Waldrun's actions.) Before he finds his father, he encounters the spirit that Auric had freed. He warns him against staying with the dead, for it is too easy to forget about life. He also warns him of the Taghairm ritual, and bids him to save Talia's life. Dain returns to Barathrum, warns Varn Gossam, and helps break Talia out of jail. Auric channels the sun into the rift and breaks down the veil between Barathrum and Creation, which blinds the Once Elf guardsmen and allows them to escape. (They would not have been able to do this, however, if the guards had not first been distracted by the portal to the Netherworld unexpectedly opening. That was caused by an invisible Rathus, returning with the Netherblade.) Varn sees Lugus in the light, and goes on to join the Malakim and found the Empyrean. The Malakim had always revered the God of Light, but he had not had a priesthood or any formal doctrine since the Calabim exterminated his organized religion over 600 years earlier. Talia stays with Varn and becomes his wife, although years later she cheats on him with Decius and is driven to suicide.
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