Elex

This achievement is related to the thieves guild "The Claws" located in the Domed City. It is kind of missable if you are not thorough enough. (Update: I checked again - you never have to go there)

There are two ways to enter the guild's hideout but first you need the entry token from Katta (Berserker). She is located on an island in south Edan and after you do a quest for her, she'll give you "Katta's Claw".
Note: Andrinho7x pointed out that you can get a claw from Rat (Outlaw) as well. He'll give it to you at the end of his quest line.

How to enter the hideout:
a) With the key from barkeeper Vito in Domed City. You can either pickpocket the key from him You can get it from him through dialog as well if you have the claw in your inventory. After that you'll get the quest "Strange mechanism" which will show you the way to the trapdoor.
The trapdoor is in the west part of the city. To find the keyhole next to a bonfire, stand in front of the trapdoor and look up and slightly to the right.

b) Access the hideout through a passage above the teleporter ("Small Farm" or similar) south of the city. The actual entrance is north east of the teleporter. It is right above the small "shed" behind the house. You'll need the jetpack to get up there. Or follow the city wall and jump down.

Show the claw to bouncer Darius and then exhaust the dialog with Hector. Achievement should pop then.