Chester's Black Zweihander

 is a weapon, presumably an Ultra Greatsword, which was removed from the final release of Artorias of the Abyss DLC.

Overview
As the name states, this weapon was intended to be connected to Marvelous Chester in the Royal Wood, either as a dropped weapon, or as a gift to the player. However, as it was cut from the final release of the game, it is unknown how the player would have obtained it.

Due to its nature as cut content, Chester's Black Zweihander is simply a texture in the game files. This means that even if the player modified their save file, it would not have its own stats anyway, as an existing weapon's model has to be overridden in order to 'obtain' it. Hence, the weapon can only be seen by those who are modifying the game to use its model.