It is Valyrian steel longsword with a black blade.
Ser Jon Roxton wielded the blade during the Targaryen civil war known as the Dance of the Dragons while fighting for King Aegon II Targaryen and the greens. During the Battle on the Honeywine, he used it to deliver a fatal wound to Lord Owen Costayne. During the Second Battle of Tumbleton, Jon slew Hugh Hammer with Orphan-Maker, splitting him from groin to throat. Shortly thereafter he was killed by Hugh's men in a ten against one fight.
The sword came into the possession of Lord Unwin Peake. When Peake became Hand of the King during the regency of young king Aegon III Targaryen in 133 AC, he drew Orphan-Maker, declaring that unlike the previous Hand, the blind and crippled Ser Tyland Lannister, he could still wield a sword.
The sword's current whereabouts are unknown.