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.)

Reward

(First Chapter)

(Second Chapter)

(Third Chapter)