Dainsleif

Dainslef (ダインスレフ) is a demonic sword from the Nibelungen, a cursed Noble Phantasm that brings ruin to its possessor. It is a demonic sword handed down the family that killed the Northern European hero Sigurd, and it is a Noble Phantasm originally possessed by the dragon Fafnir.

It has a strong curse of "revenge", but it also guides its possessor to ruin.

Demonic swords and holy swords contain both glory and ruin, but this Noble Phantasm only brings ruin to its possessor.

Source: weapon status screen, translation by Mirror Moon.

In the game, it is seen amongst the weapons in Gate of Babylon.