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. Accounts obtained by Chloe and Nanami Minor reveal that Derith actually forged Greyswandir himself, obtaining the metal from an underwater city of the Old Ones, forging the blade in an underwater volcano, and tempering it with the blood of three Elemental Princes of Water.
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 Herald Crisis. Lucien was able to retrieve the Blade of Water from her, and since then learned more applications for the blade; however, this knowledge, as well as Greyswandir itself, was lost when Wu-Lord Anno rode a Primal Chaos bomb (yes, rode it) into Rebma's army during the Hel invasion.
In 1340 AY, Prince Drake returned from Shadow bearing a new Greyswandir, presumably reforged by himself. He held into the blade for some time, using it as a carrot to get people to do things for him and as a secondary weapon. Drake traded it to Nanami Minor for the spikard Wings of the Dragon early in the Logruswrack Crisis, and Nanami still bears it, generally using it as a bladed gauntlet.