THE LAST CITY
Now that you have collected the four pearls of the Corrupted Elements, it's time to go back to The Pit, beneath the Black Mill near Gnaar Mok, and talk to Tommy. He hasn't unlocked the corrupted Shrines yet, so you'll have to come back for that later. First you need to go to the Last City; Tommy will hand you the amulet called "Unio" to give you access through the gate. There's also a Captain of the Imperial Legions to speak to, but you can do that later.
As soon as you enter the city, you're called upon to rescue a lady in distress, who's being attacked by a Ghoul. Help her kill it and talk to her; she'll take you to into the nearby safehouse to continue the conversation about the situation in the Last City. Talk to Thyrof next, he'll ask you to go to the Shrine of Health to get a Ruby of Health, so they'll know you're the real you.
THE SHRINE OF HEALTH
Head out and to the right until you see a well, then turn left. Along the way you'll encounter some skeletons. A little further you can go left again, where you'll see a Ghoul. Be careful, because there are also some Artifex Hunters around the corner. The entrance to the Shrine of Health is behind the fallen tower.
Inside the Shrine there are only some skeletons and liches to deal with, and you'll find the Ruby of Health soon enough. Make a note to remember the location of the Altar of Health, you might need it later...
THE TOWER
Return to the safehouse (Eye's Hideout) and receive your next mission, which is finding Lady Stadi Galanos in her tower. Go past the well and turn right, then continue straight ahead, past the docks and across the bridge. Deal with the skeletons, Artifex Hunters and Ghouls along the way. Approach the entrance to the courtyard with caution: the two skeletons you'll see first are small fry compared to the opposition that's guarding the front door!
If you saw many Artifex Hunters and Ghouls outside, inside it is not much easier - all Artifex Hunters, and even a Demon Skeleton (you might remember it from the Realm of Ignis). Have fun and slay them all, before entering the Safe Zone.
Of course the Safe Zone is not as safe as advertised. You'll probably hear the noise when you walk up the stairs - bugs! There are a lot of them, and it's safer to get rid of them all before continuing, because that's not all...
When you go through the door you'll be surprised and surrounded by a couple of Demon Skeletons and a Ghoul. Kill them and continue upstairs, you'll find a Demon Bonelord up there, that holds the key to the door immediately behind. Go through, and you'll see a transparent door at the other end. You're nearly there...
There's a button right next to the door when you enter. Push it and go in, you may speak to the Lady now.
THE KEY
Lady Stadi has several successive missions for you, the first of which is getting a key from a house in the nearby possessed quarter. The entrance to that quarter is a wooden gate to the left when you exit the courtyard. The Keykeeper's house is called Joba's House, and is in the far north of the quarter. Although it's more advantageous to sneak through, you can go in openly, if you want. But that way you'll have to deal with all the poor possessed people who'll try to club you. There's a Ghoul too, but you'll have to watch out more for the Whisp, which looks like a blue-lighted skull collection.
If that manages to touch you, you'll be possessed, which means you'll suddenly find yourself in the hideout, with all your clothes off and most of your attributes reduced to zero. Go to the Shrine of Health if that happens, and get your attributes restored - and your clothes and armor back on, of course. Then go back and continue where you left off.
The key you need is in the cellar of Joba's House, but beware: there's another Whisp down there! There's a locked chest which contains the Key of the Secret Tower Chamber; bring that back to Lady Stadi.
THE POTIONS
Your next mission for the Lady is to retrieve two potions from the alchemists quarter. There are several ways to get there: one way is to jump over the wall at the cemetery; another is to jump in the water, swim through the canal to the east, and then try to climb out somewhere. But this approach will make you vulnerable to the opposition (more Artifex Hunters).
It's better to go right when you leave the courtyard and walk into the water. Around the corner you can see a fallen tower in the distance, which is blocking your way into the alchemists quarter. And there's also a Ghoul, which practically stands on top of an access grating to the sewers. Get in there, and swim straight ahead for a short distance, to the next grating. You could explore the many sewer pipes, there are many enemies to practice your skills on, but I won't do that now. When you exit the grating, you'll find yourself in the alchemists quarter, right behind the fallen tower.
Which way you go from here doesn't matter much, sooner or later you'll see the Home of Max. Break the lock and go in.
Max turns out to have only one potion for you, you'll have to go to the harbor and Quistnix' house to get the other one. As soon as you finish talking, Max drops dead at the hands (or out of fright) of an Artifex Hunter - kill it. Don't forget to help yourself to some potions from Max' wooden chest before leaving.
There's no need to go back through the sewers again, you can go north through the canal which ends in the harbor. The Home of Quistnix is in the northeast. The door is heavily locked, but apparently Quistnix put a small key in a convenient place - under the doormat. Go upstairs and clean out the opposition in the bedroom. The potion is not too hard to find; look underneath the pillow. Now take both potions back to Lady Stadi.
DURO'S SPLITTER
She has one last mission for you, and then you're done training your Athletics skill running up and down the stairs in the tower. You need to go into the deepest parts of the catacombs at the cemetery, to find a magical staff called Duro's Splitter. As a reminder: the cemetery looks like a park with barren trees, and it's near a well. Go inside the catacombs through the door in the corner, and fight, avoid or ignore (whatever you like best) the many skeletons and liches down there. Go to the south to find the door to the deeper part of the catacombs.
At the end of the passage you have to go left, through a wooden door. Right behind it are two skeleton warriors; to the right is another passage; and to the left is a hidden door! Remember that, you'll have to get back here in a few minutes.
But first go right to fight a couple of Bonelords - well, one of them is a Demon Bonelord, which is much tougher, but he carries a useful Tomb Key, so it'll be worth it. At the end is another wooden door, behind which is a Ghoul guarding a trapdoor to the sewers. You don't need to go there now, but after completion of this mission you won't be able to see it anymore.
Go back to the hidden door I told you about, and downstairs you'll find another hidden door - and then another one! Behind this door you can finally find Duro's Splitter - it's the ebony staff on the corpse. Take it with you to Lady Stadi.
Hasib is with her when you get there. Just talk to the Lady, and she'll send you back to The Pit, so you can restore the Corrupted Elements.