Fort Grymmdjarr is an old ruined fortress southwest of Fyresdal in southern Ard Skellig and named after Grymmdjarr, the founder of Clan an Craite of Ard Skellig.
It is now home to a harpy nest, though it's also the temporary camp for thieves during The Family Blade, during which the harpies won't appear, but the nest can still be destroyed.