How to get the Dagger of Symmachus - Elder Scrolls: Morrowind
Location: Mournhold, Throne Room
Requirements: Complete most optional Tribunal quests to reach Helseth's Champion
Video: How to get the Dagger of Symmachus
The Dagger of Symmachus is given to you as a reward for completing the Helseth's Champion Side Quest in the Tribunal Expansion Pack. You'll receive this quest from King Helseth himself as you progress through the Tribunal Expansion and complete the main story quests, it's one of the later quests in the Expansion.
For the Helseth's Champion quest you're tasked with beating Karrod, King Helseth's personal bodyguard in a duel. There are a couple of ways you can make this duel easier if you'd like; to learn more about the quest itself or how to complete it check out the UESP Helseth's Champion Entry.
It's possible to kill King Helseth and loot the weapon from his corpse as well without ever having to complete this side quest. If you plan to kill him for the Royal Signet Ring anyway then you can (literally) kill two birds with one stone this way.