The Falconer was a Bondsmage from Karthain in service of the Gray King. He assisted the Gray King in his scheme against Capa Barsavi and during that scheme came on the wrong side of the Gentlemen Bastards, of which he helped kill three and later got punished by the remaining two, leaving him mad and unable to ever again practice his art.
History
Republic of Thieves
In the Republic of Thieves we learn that the Falconer's mother is Patience, one of the leaders of the Bondsmagi. It is rare that the offspring of magic user are magic users, and the Falconer was one of these. His father died when he was young, leaving him to be raised by his mother. In an attempt to connect with him, Patience gave him his true name. This was a terrible thing, as knowing someones true name allows them to have power over them. The Falconer resented, then grew to hate his mother because of the power she held over him.
The Falconer was a speaker for one of the factions of the Bondsmagi. He believed that magic should be used to rule humanity. However, his mother believed that such a use of magic would call down the doom of the Eldren. After the destruction of the Black Iris faction at the end of Republic of Thieves, the Falconer is the only one still alive.
After the Bondsmagi disappeared, Patience once again visited her son. She revealed that she caused him to go mad after his mutilation, and left him behind to live out his life without his tongue, fingers and gifts. However, the Falconer used dreamsteel to create new fingers and a new tongue. With his magic restored, he possesed a murder of crows and used them to murder his mother. He then leaves the city, to continue his objectives of ruling humanity, and gaining revenge.