Upon reaching level 100, a quest will appear in your Lightbulb notifier on the left column. Select the quest“[Dimension Library] Visit the Dimension Library” to proceed.
Access to ‘Grand Athenaeum’ via the Dimensional Mirror.
Complete all 3 chapters. (Note: You can always exit the portal and resume at anytime.)
You will get The Bookworm Medal, a pendant, and an extra pendant slot for 20 days. You can expand the expiry date by 7 days once you complete the additional quest from the NPC in Dimension Library.