It's right in the cave (like ... it's small) in the Waterfall near Alexander, you can only access it by flying.
Usually if you stand at the Camp where Mide (NPC of the Alexander Story) resided for that Part, the Cave is located in the East in the Dravanian Hinterlands.
https://i.imgur.com/aZrFzPL.jpg
This one shouldn't be hard, I mean... ya... still one of the few Spots in the Game that kinda gives you a rather good feeling.
(Just continue, I'll be heading to Work so responding is unlikely ... *scratches head*)
https://i.imgur.com/srUI4Qm.jpg