Known alternatively as the Shade Blade and the Blackrock Sword, the Black Sword was an artifact of great power forged from the indestructible substance known as blackrock. Forged on the Isle of Fire by the Avatar, and wielded thereafter, the artifact was lost after the Avatar's adventures on Serpent Isle came to an end.
This mod not only adds the Black Sword, and the ability to make it, it adds blackrock as well to forge it from. Starting at level 18, you can find it rarely on Falmer and in Falmer chests, uncommonly on Dwarven Spiders and Dwarven Centurions.