What shocked Lin Yun was that this place was too special. In the spacious decaying library, he had read the diary left behind by Starry Sky College's Wayate. It recorded Wayate's later years and contained all his experience, including how he led the Starry Sky College's Planar Legion to the depths of the Volcanic Mountain Range and ended up being besieged by a large amount of Raging Flame Beastmen. At the time, he scattered the Planar Legion and luckily hid away without being found. After exploring, he ended up discovering the Book of Ten Thousand Mantras.
As for the details regarding how he obtained it, Wayate hadn't mentioned it in the diary. He only vaguely said that it was in the Volcanic Mountain Range.