Herebane's Fortress is a unique Elven shield which was cut from The Elder Scrolls V: Skyrim before release.
Acquisition
The item cannot be obtained without using console commands.
Background
It was supposedly from a person named "Herebane Sorenshield," but this person was cut from the final game. He, his shield, and his sword can still be found inside the Skyrim's files.
Smithing
It can be upgraded with a Refined Moonstone at a workbench and also benefits from the Elven Smithing perk, which doubles the improvement.