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.
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.