Hieronymus Lex is a Guard Captain in the Imperial City. He has sworn to find and arrest the Gray Fox and his weapon of choice is a Silver Claymore. He is usually located in the South Watch Tower accessible through the Temple District and the Arena. He is the main antagonist of the Thieves' Guild questline.
While exploring the Temple District in the Imperial City, Ruslan or his friend Luronk gro-Glurzog, may ask for a few septims to hold them off until payday. Whether or not money is given to them, they will explain that while at Jensine's "Good As New" Merchandise a captain of the city guard came in and accused them of theft. Despite their innocence and Jensine defending them, the guard told them to hand over their money or face time on the Imperial City Prisons.
It's suggested that the Hero begin this quest right after they talked to Christophe, or between 12:40 and 1:10 on Tirdas, as this time gives the easiest entrance. Hieronymus Lex is located in the South Guard Tower accessible through the Temple District and Arboretum.
Armand Christophe gives the Hero the task of stealing Llathasa's Bust and sends them to Cheydinhal. Travel to Cheydinhal and speak with the beggars marked on the map, if their disposition isn't high enough, do whatever is needed to rise it until an option to ask about the bust appears. They will then tell the Hero to look in the chapel.
Talk to S'krivva in Bravil and she will tell the Hero that Hieronymus Lex has pulled guards from all over the Imperial City to the Waterfront in an attempt to root out members of the guild including the Gray Fox. The Thieves' Guild must stop him at all costs or the guild may be finished for good. She will then tell them to meet with Methredhel in the Imperial City.
In the quest Taking Care of Lex, Lex is transferred from the Imperial City to Anvil to work as a guard captain of Anvil. He hopes that fate will bring the Gray Fox to him. In the end, when the Hero becomes the Gray Fox and wears the cowl, Lex confronts him with two others. The Hero's choice is to decide whether Lex dies or not.
A copy of the Black Horse Courier states that a man was arrested for claiming to be the Gray Fox, the Reincarnation of Tiber Septim, a Were-shark, the love child of Stendarr and the mother of Hieronymus Lex.
- Lex in Latin means "law", possibly referring to his status as a guard captain and the tradition of Imperials having Latin names.