This is a MUST have mod for anyone who loves enchanting! The cave can be a little tough to find at first, as it doesn't have an icon that pops up when you read the "Letter to Balgruuf." However, if you just leave Whiterun and walk around just outside the castle walls, you will find it on the north side of Whiterun. Don't go looking for a "North Watch Tower," it doesn't exist. The creator meant the northernmost watchtower that was part of Whiterun's castle wall. The necromancers are a tough bunch to kill at first, but if you bring a follower with you then you should survive. Save before you go inside as they attack pretty much as soon as you enter the cave. Great mod! VERY USEFUL!

They get the soul gem from mining inside the Black Reach