How to get Shadowsting - Elder Scrolls: Morrowind
Location: Treestump behind Thirsk Mead Hall
Requirements: Able to open a lock level of 20
Video: Location of the Hollowed Treestump behind Thirsk
Shadowsting is one of many items that you find in a Hollowed Treestump behind the Thirsk Mead Hall on Solstheim. Inside of this container you'll find 5 enchanted items which vary in usefulness depending on your character's build along with a vague note. The best items in the container are arguably Shadowsting, Ebony Arrow of Slaying and the Ring of Raven Eye.
Thirsk is a small town found just east of Lake Fjalding on the island of Solstheim; the easiest way to reach this location is to travel directly north of Fort Frostmouth until you hit the river then go east until you reach Thirsk. If you need help locating Thirsk use my screenshots below, it's called a town but it really is just one large building east of Lake Fjalding. You'll find the Hollowed Tree Stump just behind this building, it's very difficult to miss.
Items inside the Hollowed Tree Stump:
Shadowsting (Long Blade)
Ebony Arrow of Slaying
Ring of Raven Eye