Widow's Wail is a Valyrian steel longsword.
The blade has red and black ripples through the steel and its scabbard is garishly decorated with gold, cherrywood and red leather with golden lions' heads. The eyes of the lions are rubies.
A Storm of Swords
Lord Tywin Lannister tasks Tobho Mott with creating two new blades from the late Lord Eddard Stark's greatsword, Ice, with one for Tywin's grandson, King Joffrey I Baratheon, and one (Oathkeeper) for Tywin's son, Ser Jaime Lannister.
Tywin gives one of the swords to Joffrey before his wedding, replacing Hearteater. Accepting the name Widow's Wail when it is shouted by a guest, Joffrey uses the blade to destroy Lives of Four Kings, a gift from Tyrion Lannister.
Upon Joffrey's death, Widow's Wail passes to his younger brother, Tommen I, and is held at the Red Keep until the boy king is old enough to wield it.
Most Valyrian steel was a grey so dark it looked almost black, as was true here as well. But blended into the folds was a red so deep as the grey. The two colors lapped over one another without ever touching, each ripple distinct, like waves of night and blood upon some steely shore.
– Tyrion Lannister
Widow's Wail! Yes! It shall make many a widow, too! And when I face my uncle Stannis it will break his magic sword clean in two.
– Joffrey I Baratheon
to his guests
Addam: Have a care, Your Grace. Valyrian steel is perilously sharp.
Joffrey: I remember. Sharp! I told you, I am no stranger to Valyrian steel.
– Addam Marbrand
and Joffrey I Baratheon