The Thieves Guild skill-line is part of the "Guild" category in the Elder Scrolls Online. The Thieves Guild skill-line only has passive skills. The Thieves Guild focuses on improving your criminal pursuits such as fencing, bounties, or decreasing your detection range. The Thieves Guild can be found in the Hew's Bane zone in the Abah's Landing city.
How do you unlock the Thieves Guild skill-line?
To unlock the Thieves Guild skill-line you have to join the Thieves Guild, which is only possible with the Thieves Guild DLC. Another option you have is to open a treasure trove, that will also grant you the Thieves Guild skill-line.
How do you level up the Thieves Guild skill-line?
To level up the Thieves Guild skill-line in the Elder Scrolls Online you have to complete Thieves Guild related quests.
When a guard accosts you, you may use Clemency once per day. If used, the Guard will not arrest you or take your money and stolen goods. Additionally, Guards will not attempt to accost you for 1 minute after you use Clemency unless you commit other crimes.
Decreases detection range of Witnesses and Guards by 10%. Witnesses and Guards are thus less likely to notice criminal actions, though this has no impact on the range from which Guards will accost you.