Available during "Fall of the Corrupted Eunuch" which happens during Part 3, at a recommended level of 29. You can always revisit this area via Travel.
It is advised to take care of most of the enemies in the first area. In order to grab this, you'll want to drop down to the bottom level, go to the far back left room to loot the dungeon keys. You can then use the cells to mantle back up to the top level back to the entrance. Here, use the dungeon keys to talk to the man inside, and exhaust all the dialogue options to unlock it.