How to get the Stalhrim Longsword of Flame - Elder Scrolls: Morrowind
Location: Skaal Village, Solstheim
Requirements: Completed all Bloodmoon Expansion quests until you reach The Castle Karstaag
The Stalhrim Longsword of Flame is your reward for completing The Castle Karstaag Story Quest in the Bloodmoon Expansion Pack. In order to reach this quest you'll need to complete the majority of the Bloodmoon Expansion pack as well as choose the path that doesn't involve your character becoming a werewolf.
If you choose the path that doesn't involve becoming a werewolf then this quest will be the penultimate (2nd to last) quest of the expansion. Choosing to become a werewolf will give you a different set of quests and will require you to forsake getting this weapon.