The Sorcerer is an unnamed minor character in the third season. He appears in "And Now His Watch is Ended". He's a sorcerer established in the Free City of Myr and the man who made Varys into a eunuch, having taken his sexual organs for a magic ritual.
|Season Three appearances|
|Valar Dohaeris||Dark Wings, Dark Words||Walk of Punishment||And Now His Watch is Ended||Kissed by Fire|
|The Climb||The Bear and the Maiden Fair||Second Sons||The Rains of Castamere||Mhysa|
In the books
In the A Song of Ice and Fire novels, the sorcerer is mentioned long before by Varys when he tells Tyrion of his origins, while they were discussing the strange circumstances of Cortnay Penrose's death. There's no mention, however, of Varys ever tracking down the sorcerer to exact revenge.
So far, the books never even made it clear if Varys' tale about how he was castrated by a sorcerer was actually true, or just a story he fabricated to confuse his enemies. On the other hand, it was never implied that he was lying, but Varys is such a master of deception that it was difficult for readers to trust any story he gave about himself. The TV series depicted Varys showing the captured sorcerer to Tyrion Lannister, confirming that his story was true.