You need to do all the Land.Of.The.Morning.Light "Collection of tales" from "Morning Light I" (page 1) until "Morning Light II" (page 3) where you'll start the first main quest chain called "Tale of Chunhyang", you will get the knowledge "Im Yeook" automatically by just following the quest chain until the small town called "Yuunru", just follow the quest!

PS. Accept the quest via black spirit main quest section.
Type "Collection of tales" in the menu's (ESC) search "Enter a search item" to find it.