Kill more ashen owlbears until you have found 8 owlbear feathers. Although the quest helper shows the greater spawn area for the ash owlbears, they are primarily concentrated around ( -563, 43, 139 ) /waypoint -563, 43, 139.
Find Dannian Blackleaf ( 426, 26, -1510 ) /waypoint 426, 26, -1510 near the waterfall in Nektulos Forest.
He randomly spawns and despawns for days on end (game time). He does NOT appear on a regular cycle based on observing his location for 3 days game time.