Coin is a main character in the novel Sourcery. He is the eighth son, of an eighth son, of an eighth son. He is therefore a Sourcerer, a Wizard-squared. He is currently the only known Sourcerer in existence.
Coin was born a Sourcerer. His father, Ipslore the Red died shortly after his birth, but imprisoned himself in to a staff to cheat Death. He left the staff to Coin and through it he was able to control and protect him.
While still a child, his father coerced him to travel to Ankh-Morpork to assert himself upon the Wizards there as Archchancellor of the Unseen University. After an effective demonstration of his power the other Wizards reluctantly agreed to make him Archchancellor.
The Wizards were able to feed of the Raw Magic he generated and thus become more powerful than they had been in centuries. The Wizards used this new found power, under Coin's leadership to seize control of Ankh-Morpork. During their coup they transformed the Patrician in to a lizard.
Not content with ruling Ankh-Morpork Coin set his sights on the rest of the Discworld. He was initially opposed only by Abrim the Grand Vizier of Al Khali who was himself being controlled by the Archchancellor's Hat.
In the aftermath of the ensuing magical war Coin realised he was being controlled by his staff and threw it away, breaking the spell which kept his father contained within it. Death promptly appeared to claim his father and Coin was freed of his control.
Seeing the damage he had done, Coin used his powers to return the world to how it had been before his interference before creating a special world to stay in.
Coin was the eighth son of Ipslore the Red a disgraced wizard.