The Library of Fate houses tomes of history and a vast number of gems that are reputed to be shards of the Crystal Arch. It is through these gems that Talus'ar's writing is able to change, based on the information that Itherael seeks. Within the library, angel ascetics peer into the crystals, recording the visions they behold for Itherael to interpret. Some records suggest that these crystals provide multiple visions of multiple futures, for when Anu (from which the Crystal Arch comes) was blasted apart, his state of all-knowing was fragmented as well.
During Diablo's invasion of Heaven, the demon Rakanoth held Auriel here. However, she was rescued by the Nephalem. After Malthael's defeat, Itherael discovered that tomes within the library were changing the Heavens themselves, transforming into small pockets reflecting the past, present, and potential futures. Though none could make sense of why this has occurred, it still posed a threat to Sanctuary. Tyrael invites the Nephalem to investigate. In truth, the battle between Diablo and the Nephalem previously had somehow sparked this event.