This is far too big to be called a sword. Massive, thick, heavy, and far too rough, it's more like a heap of raw iron.
You gain the enormous sword of the Black Swordsman, Guts. It is a one-of-a-kind piece that was forged by the blacksmith Godot, intending to create a sword truly strong enough to kill a dragon. What he made was unusable by a human being, or so he thought.
You also gain the strength and skill of the only man to ever successfully wield this blade, Guts. This includes the strength to fight Apostles on equal terms, speed that can't be seen by normal human eyes, and the skill to kill 100 knights in a single night. Part of this comes from the Brand of Sacrifice on Guts' neck, which places him in between this world and the next, allowing for mind to triumph over matter & for Guts to detect supernatural beings and events by feeling their effect on the brand. You gain these abilities without the curses that come along with it.
It is unbreakable and will never lose its edge. Thanks to being bathed in the blood of astral beings over and over again, It is able to harm not only physical beings, but supernatural and metaphysical ones, such as the God Hand.