One of the five elemental Patternblades, Greyswandir is the blade that was recovered by Prince Derith? under unclear circumstances during his studies of the advanced arts of the Pattern.
Greyswandir is a grey hand-and-a-half sword with a faintly-glowing blue tracery upon it, depicting a portion of the Pattern. Its crossguard is made of a grey metal, engraved with stylized depictions of the ocean waves. The grip is brown wire-wrapped leather. However, this description is somewhat dubious, as the Blade of Water is the ultimate shapeshifting weapon, capable of changing itself into any (realistically) imagined weapon or item that could be wielded. It also has the ability to detect and reveal shapeshifters. What other shapeshifting-related abilities it might possess have yet to be discovered. Greyswandir also has the usual suite of Patternblade abilities -- high damage value, spellbreaking, setting Chaos blood aflame, etc.
Greyswandir was one of the two Patternblades (the other being Werewindle) stolen by Annadil to open the Gate of Revolution on the Second Day of Darkness. After the Gate was closed, the two Patternblades vanished into Shadow, presumably returning to their owners. However, Greyswandir somehow ended up in the hands of Bronwyn, a Shadow warlord and one of the potential Herald candidates, between the Second Day of Darkness and the present day. Lucien was able to retrieve the Blade of Water from her, and has since then learned more applications for the blade; however, this knowledge, as well as Greyswandir itself, were lost when Wu-Lord Anno rode a Primal Chaos bomb (yes, rode it) into Rebma's army during the Hel invasion. The Night Blade will presumably be re-forged, but by who and the methods of how are unknown at the moment.