How do you get to the location south of "The Castle of Light"?

I just finished the Lux quest.

On the map there is an undiscovered area below “The Castle of Light” that I can’t find the entrance to. Only ways out of The Castle of Light for me is “The Unenlightened” and “Metal Fields”.

Holding “Q” on the keyboard to get the “Zone hints” doesn’t show anything either.

Has anyone found a way to the southern location?

It becomes available once you forward the main quest line.

Thank you!!