This is a quick little mission for the thieves guild. You need to pass their obstacle course and officially join before they offer it though. The competing guild, the dark eyes, managed to steal Count Growin's hammer from the thieves during their last mission. So we need to get it back.

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You can only take one person with you. This means you have to send your combat people back and just hang on to either Gladys or Dranor. Go over to the marked hideout for the dark eyes. Talk to your old friend outside to start the mission. You should probably save just in case. It's possible to fight your way out, but it's really hard and not too good. We'll do it the right way.
You don't need stealth skill for this though. Just follow these instructions. Start sneaking and pick the locks on the chests by the door. You might as well get a few dollars while you're at it. Then go up to the big chest in the shadows to the left. Sneak up just enough to trigger a cutscene that will make the drunk thief wander toward you. Don't worry. He's so drunk that he'll always mistake you for a fellow member. When he walks past you, he'll trigger the ladder.
Use the ladder to get to the upper level. You then have to break the wooden boards that make up the little bridge. That starts a distraction for the one sober lieutenant and the 2 men on the floor. Move over to the ladder and drop down. Make sure that you're still in sneak. Pick the lock on the chest on the counter. You can then grab Growin's Hammer. Then just go up the ladder and backtrack out of the hideout.
You'll automatically turn it in for your reward.