You will need to find a ??? in the caves at around (F-8), but it cannot be reached directly from the entrance. Once in Uleguerand Range, you'll need to make your way counter-clockwise across the zone, slowly climbing to the top of the Thousandfall Ridge.
Avoid Jormungand, a HNM which spawns in your path and also has true sight.
Once you are at the top, start walking towards (F-8) and you will see two slopes. One to the south and one to the west.
If you slide down the southern slope, you need to land on a small ledge leading to cave S3 on the map of the Southern Slope to the right.
If you slide down the western slope, you need to land on a small ledge leading to cave W2 on the map of the Western Slope to the right.
If you fall into one of the holes on the slopes (S1 or W1), you must go back onto the slope and land on the proper ledge.
Once you start down a slope, you're committed. You can steer a little to the left and right and you can slow your descent by running against gravity. If you miss the outcropped cave ledge, you will have to climb back up the entire mountain.
It is possible to have Tractor cast on you to put you on the ledge should you miss.
S1 and W1 lead to the black tunnel path which can be seen in the cave insert on the Uleguerand Range map to the right. Caves S2 and W3 lead to the white path. The ledges you seek (S3 or W2) lead to the red path and the ???.
When you reach either S3 or W2, follow the path to the ???.
There is also a hole above W3 that's leads to the W2 path. There is an eye at the bottom of the hole so sneak or silent oil is recommended before going down.
Trade in the Moblin Hotrok for a cutscene and to complete the quest.
You will receive a different reward if you already have bought a Map of the Uleguerand Range. This is also reflected in the dialogue, which will mention "a reward" rather than the map explicitly.
Antreneau's Uncle Louverance set out on a journey to cross the Uleguerand Range quite some time ago. Antreneau tells you that the other side of the mountains can be reached through a cave found on the slopes of Thousandfall Ridge...
SPOILER WARNING: Click here for additional details about this story line.