The Library of Barathrum contained many priceless manuscripts which the Once-Elves took with them when they escaped the Netherworld. These are the greatest pieces of literature ever written, masterpieces whose authors were not able to complete during their lifetimes but perfected after death. Auric was supposed to have learned the details of Mulcarn's summoning, rule, and demise by reading a posthumous account written by a High Priest of Winter who saw it all happen first hand. This could not have happened, however, because the Once-Elves left the Otherworld before Mulcarn's death and have never returned for more manuscripts. Auric Ulvin was born within one year of Mulcarn's demise, and was 16 years old when he found Barathrum. Talia Gossam says that the Once-Elves founded it after escaping from an evil archmage who had held them in the land of the dead for centuries, and that they had been living in this city for decades.
Although not mentioned in the story, Kael's reply revealed that Rathus Denmora was in Barathrum at the same time. Sandalphon had visited their library a generation earlier, where he found The Book of Laroth. (This is one of the most powerful books of magic ever written, and also most likely a very biased history meant to manipulate the emotions of the reader in order to serve Laroth's purposes. I very much doubt that Laroth revealed his true identity in it. He probably pretended to revere Arawn, rather than desire to overthrow him. The Sidar would be appalled if they knew his true plans.) The Once-Elves would never let a visitor remove any original manuscript from their library, but they let guests read and make copies of anything they like. Sandalphon made a copy of The Book of Laroth and took it home to his people, who used a ritual found within to become the first Shades. Rathus had already become a Shade before he realized that Sandalphon's copy was of poor quality, riddled with errors that made its spells far less effective than they should have been. He came to Barathrum to study the original and bring back a corrected copy for his people. He entered the city at the same time as Gosea the Dwindling and Talia. He was still there when Gosea the Dwindling stole the Heartstone and escaped. Harlond Gossam (due to the influence of his chief adviser Waldrun, a necromancer who is secretly in league with Laroth) blamed Talia for the theft. Rathus would have been a good scapegoat too, but performing the Waning ritual perfectly made them forget he had ever existed. Waldrun tried to convince Haerlond that the children were working with Talia to hide the truth of the theft of the Heartstone. They spent one night in jail before Varn convinced his brother to release them. (While there, Talia recognized Auric's aptitude for magic and gave him his first formal magic lesson. He was disappointed with his slow progress, but she was impressed. He had advanced more in his first few hours than she had in her first several months, and her teacher's had considered her a great prodigy.) While Auric was in the library, Waldrun tricked Dain into visiting the Otherworld--and thus, by Once-Elf Law, never being allowed to leave their liminal realm in the other direction and return to the land of the living. Rathus used this diversion as an excuse to visit Arawn's vault himself. There he was given The Netherblade by an angel in the service of Laroth, whom he believed to be a servant of Arawn. (When Auric learned where Dain had gone, he tried to sense his presence using magic; instead he made a brief but intense connection with someone overwhelmingly powerful and evil. It is strongly implied that this is Laroth himself, and it would not surprise me if the Master of Spirit magic used this opportunity to plant the seeds of the madness that would drive this nice young man to become a malevolent deity. He probably planed both Auric's rise and fall, as steps towards his own deification.) Dain returned first, as a departed spirit warned him that Waldrun intended to use the taghairm ritual to kill Talia and enslave her soul. Haerlond had agreed to this, as it would ensure she would confess. Dain returned to Barathrum to warn Varn, who helped them and her escape. Rathus did not return yet. His return was the mysterious distraction that came just in time to stop the Ostaurii Guards from killing Auric before he could finish channeling the Sun, blinding the enemy, and breaking down the barrier that had separated The Vale of Shadows from Erebus itself. It did not come in time to save the life of Dain, but it let Varn, Talia, Auric, and Kirien escape.