Mists Take You

Cave Entrance is at 52.2, 79.7

The easiest way to reach it is probably flying to Ironhorn Enclave and then jumping down the mountain with Slow Fall/Glider

/way Highmountain 52.2 79.7

Don't be fooled by your map. There's actually 2 cave entrances down in the valley. Do NOT go into the one that's visualised on your map; the one that's called "The Deep Roads" when you enter, it's not where you want to go. The actual cave entrance you want to go to is directly under the quest marker and isn't visualised on the map, it's called "Echoing Halls", go in there.

There is a simple way to get there from Ironhorn Enclave without needing a glider or slow-fall or going a great distance.

From the Ironhorn Enclave flight point (56,83), face north. Straight ahead you should see about a hundred yards there is a wooden totem shaped like a bird with a couple of kites flying from it. Go to it (ignore the bird on the stand that marks the flight point). That wooden totem (at 56,81) to the north marks a trail around the mountaintop. Follow it up and around. On the upward slope, there is a bridge (52,83) that goes to the next peak. Go across, then down that hill to 51,75, then go off the path to the right and look down for a bridge coming out of the hill. Drop down to 53,76 to a bridge across the chasm. From there you can drop safely to the beneath, very near the entrance to the cave.