It was the spiritual item Ascalon.

 It was 3.5 meters long and weighed over two hundred kilograms. The sword had the same effects as the sacred sword that appeared in a story based on a real legend and a magician had calculated out all the necessary values and created that sword that could theoretically slay a fifty-foot dragon.

The sharpness of the double sided sword was not uniform. The thickness and angle of each part were adjusted so it could also be used like an axe, like a razor, or like a saw. It also had a can opener-like spike and a wire close to the blade like a coping saw on it which made one wonder if the magician who had made it had been drunk. Scales, flesh, bone, muscle, tendons, fangs, claws, wings, fat, organs, sinew, blood vessels, nerves. It seemed to have truly been designed with the idea of being able to cut through every part of a dragon using that one device.

Toaru Majutsu no Index vol. 18