The Septim heir has another task for you. You have to get Great Welkynd Stone from Ayleid ruins Miscarcand, west of Skingrad.
On the first level, go towards the point indicated on your map. When you reach the big room at the end, jump down (A) and go right. Follow the corridor and go up the stairs to reach a button (B). Use it to open the door. Stand on a raised plate (C) and finally go to the next level, Sel Vanua (D).
This one's pretty much the same. Push the button (A) and go lower. At B there is a switch, you can use it to get yourself a Varla Stone (C).
Morimath. The Great Welkynd Stone can be fund at A. You can go back up using the same path as before or defeat a lich (B) and use a shortcut (C).
HINT: There's a lot of Welkynd Stones in these ruins, collect some of them as they get useful in some sidequests.