Well, it is a sword that makes undead explode. It's Deadra was the Deadra of light and dusk or dawn I think.
Dawnblade or something like that.
Not all that useful
Dawnbreaker, and it's from Meridia, who I had never heard of before before I found her orb in a chest. Naturally, the first thing I tried to do as soon as she started SHOUTING IN MY HEAD USING THE ORB was to toss the orb on the ground, but it had tagged itself as a quest item and couldn't be dropped.
Anywho... The sword must count as daedric. I tried to improve it with 82 smithing, and the ebony smithing perk (it requires 1 ebony to improve), but it only let me improve it to exceptional when it should have gone to epic if it was ebony. It's lighter than my glass sword, but also does less damage (of course, that's improved to epic). I'm actually going to try it against undead and see how it works.
On slaughterfish: Have you tried letting them attack you? I've gone in the water and floated there for a while with several slaughterfish to practice my armor skills. The Skyrim variety must be weak compared to the ones that existed in the previous games. Of course, in Morrowind, you'd be liable to get ambushed by a Dreugh if you sat around in the water for a while.