Gram

The demonic sword, the sword of the sun, Gram. It is a demonic sword of ruin and glory that Sigurd, the greatest hero of Northern Europe, possessed in the Volsung saga.

It is the model of Caliburn in the legend of the king Arthur. As Caliburn was a "sword stuck in the stone that choses the king", Gram was also a "sword stuck in the tree that grants authority" of the Volsung saga.

The legend of Gram and its possessor Sigurd is a story filled with glory and ruin, as one would expect from a great hero.

"Der Ring des Nibelungen", composed later in Middle Ages Germany, is the story of a knight equal to the legend of king Arthur, and the sword Gram changes its name and appears in it as Balmung.

As the "strongest demonic sword" equaling the "strongest holy sword", it even possesses the special characteristic of "dragon slayer", hence it is a natural enemy of the king of knight who is the incarnation of a dragon.

Sources: the Tsukihime Laboratory entry and the weapon status screen, translated by Mirror Moon.