The last room is fairly straightforwards in terms of the encounter- in the center of it, a Dwarven Centurion Master, a particularly doughty specimen of the design, is waiting in its charging dock. You can activate it by attacking it from a distance, or fully entering the room while not hidden- which you choose is up to you. Once you have made your presence known, the thing will unhitch from the steamworks and charge you in glorious combat! If you have stepped into the room unhidden, a Dwarven Sphere Guardian will join in as well, but such an enemy is of no real consequence next to the Centurion Master. The Dwarven Centurion is a huge and highly durable enemy, equipped with a hammer and axe that it will use to mash and mutilate you into a fine paste faster than virtually any other melee enemy you can find who isn’t a giant. Just to make things worse, it will occasionally vent burning steam at you in a large cone for several seconds, likely doing horrendous damage to any Followers you have along at the same time.
Since poisons won’t work on the machine, stealthy archers are a bit out of their element here and will need to pelt the Centurion Master with arrows from as far away as they can, hoping to ruin it before it can finish its slightly clumsy sprint towards the doorway. Mages can do a certain amount with frost spells to keep the thing away, but anyone not an archer or wizard needs to be spry on their feet and quick with their potions to survive the battle. Getting behind the Centurion is a bit of work, but well worth it as the thing turns rather slowly, and it’s fairly easy to hide from it behind the room’s features so that it has to slowly march around them. The Sphere Guardian makes this a touch more difficult, but can be taken out in safety if your Follower is tough enough to serve as a distraction, or you are quick enough to run around a pillar out of the way of the massive siege soldier.
Once the Centurion and its companion are down, scour the room and scour it hard- two of the platforms here can only be reached by crossing steam pipes, and these hold the majority of the Dwarven Oil and Dwemer metal that can be smelted into ingots, as well as a partial set of Dwarven armor and a Dwarven Greatsword. A small room behind a lever-operated gate holds a chest and more scrap, and Mjoll’s sword can be found in a splatter of blood beside the Centurion’s steam-power dock.
If you happen to need the use of a Dwarven device for the sake of the game’s main quest, you can also open a gate here to find one, allowing you to use the Attunement Sphere to access Blackreach. Coming here has a secondary purpose as well- since you can pick up a Dwarven Greatsword here, it may bring you to the attention of the Dwemer-studying scholar of Markarth, Calcelmo.
With the sword in hand, return to Riften and show Mjoll your prize. She will thank you for retrieving her blade, and hence be available to you not only as a Follower, but a marriage prospect. Aerin is concerned about his friend’s willingness to go out and muck about in dungeons, but she points out to him that she doesn’t need to be here to try and help the situation in Riften for him- he can do so himself with enough effort and training on his own part.