Join the Thieves' Guild is a quest given by Yergius in Lawenilothehl.
Requirements[]
The PC must be of neutral or chaotic alignment to receive this quest. The PC must also have the Pick pockets skill prior to accepting this quest, or have robbed a shop somewhere.
Description and walkthrough[]
Yergius will provide training in the Pick pockets skill for PCs who do not have it and have not robbed any shops, and then ask them to "prove their valor". To do this, the PC must successfully apply the Pick pockets skill on 20 monsters. Successful applications will either steal an item, or generate a message that reads "The monster does not have anything of value".
The PC may also rob stores to satisfy the requirements for this quest; however, this option is much more dangerous, especially for low-level PCs. However, in 2.x.x, it is often enough to rob just one shop (whether number of items stolen has influence needs testing), which can be easily done by a PC who started with potion of invisibility.
If the PC shifts to lawful alignment during the quest, they will be unable to receive the reward from Yergius until they turn neutral or chaotic again, since Yergius will refuse interactions with a lawful PC - even if they have otherwise completed the quest objectives.
Reward[]
Once the quest is completed, Yergius will make the PC member of the Thieves' Guild, which will reduce the prices at the Black Market and increase buying offers. Yergius will also provide PCs training in Climbing, Detect traps, Disarm traps, Listening, Pick locks, Pick pockets and Stealth, for a fee. In most cases you'll want to spend your money on Detect traps and theoretical training in Climbing. Prices for each skill can be found here.
Becoming a member of Thieves' Guild also has a downside: shop items become more expensive as the shopkeepers raise prices for thieves to compensate for any protection money they might need to pay. The Black Market is an exception to this rule.