There's a ladder on the building on the hill in castle ruins in serendia. Climb up it and you will find a soldier, and next to him, a hawk that gives the knowledge.

It seem that this knowledge is unlocked only when u have choosen and completed the few quests after:[Crossroad] Silence, We're Done Here!
https://bdocodex.com/us/quest/21201/1/?sl=1 (Serendia main questline)
Need confirmation tho