Sleeping Tree Camp

It can be found southwest of Fort Greymoor, or south-southeast of Swindler's Den. It features a pond with a glowing tree growing from the middle of it where Sleeping Tree Sap can be harvested. Nirnroot can be found on the Western and Southern edges of this pond, while other Alchemy Ingredients can be found around the pond.

Sleeping Tree Cave can be found along the western perimeter of the camp. A possibly hostile Giant, as well as the corpse of Ulag can be found inside the cave. It contains Ysolda's Message, which triggers a miscellaneous quest.

To the south-east, there are Nord runes with rotating stones (similiar to the ones in Bleak Falls Barrow).

The combination to open the grate is Whale, Bird, Snake from west to east. These should be facing the center of the circle (look for the arrow at the base of each pillar to see which side is the active side). The larger stones next to the rotating stones show the solution, though two of the plates have fallen to the ground nearby the larger stone, but if you search adequately you will find them and discover the solution.

After the grate in the ground opens, go down the stairs to find a skeleton and a chest with the skillbook "A Hypothetical Treachery." Reading this book will increase your Destruction skill level by one.