Harkon's Sword

Harkon's Sword is a sword that resembles an Akiviri Katana, it absorbs 15 Health, Stamina, and Magicka from an enemy if wielded by a vampire. It's apparent base stat for damage is about 19 more or less (I actually have no idea because my one-handed is maxed and it was still at that Dam). You get it from defeating Harkon in Kindred Judgment. It appears to only be useful if a vampire uses it, so if you chose the dawnguard side then i suggest giving it to Serana, I doubt she'll use it because she seems to only use daggers, so other than that idea you can ask her to make you a vampire and then maybe use it then.