Enter the building from the back and go through the mannequin storage room until you get to two guards. Let them frisk you.
Once you are in, find the nice lady in the green dress and talk to her.
Follow her to the elevator and then her room.
It may be worth remembering the enviroment. You'll need to escape later.
Once in her room, take the gun and head out.
You will be unnoticed at first. Grab & execute enemies that haven't noticed you to keep it that way.
Eventually, the "Security" force will notice you and rush either-way. Most will be melee so take them out before they reach you.
After proceeding through the brothel, you'll come to an impossible situation. An electrically shut door covered by an indestructible lamp. It's the first scenario where your companion becomes mandatory.
You'll be granted both a first-person view and control of your companion. proceed through the airducts.
The game-play for your companion will be heavily reliant on stealth. Take enemies out from behind (and from the front) by pressing Q once they are unaware of you or stunned.
Stay away from the light, press (Backspace) if confused where to go and climb up any green-shining objects you see.
Proceed on the pannel-ceiling, avoiding the holes.
Go through another short section of air-ducts and jump down.
Execute the lone enemy..
And destroy the electrical wiring.
Once controlling Jackie again, destroy the casing and proceed through the door.
There will be weapons on a nearby table. Pick them up.
There will also be a shrine. I recommend using it due to the lengthy battle-sequence ahead.
Once you killed a few enemies, there will be a rather obvious artifact ahead. Pick it up.
Proceed through the building. Enemies will be heavily armed so don't forget to eat hearts and execute.
There will be a few electrical generators ahead. Destroy them on sight.
If you are surrounded by light, follow the yellow cables to the floor to find the generators.
Eventually, you'll come into an ambush, and no matter how much enemies you'll kill, the light will blind you utterly and stun you followed by a cutscene and you meeting this game's antagonist.
You'll eventually be given a "choice" during the cutscene. Do nothing, and the choice will be random.
After that, you will be knocked out and be transferred to another dimension. Destroy the series of purple-shining walls infront of you.
Once you wake up, Press E repeatedly to "Resist".
Destroy the nearby light and get off the cross.
When you wake up once again, destroy the nearby door and head out the building.
The entire brothel will be ablaze. The fire will cleary indicate where you can and can't go. Use (TAB) if utterly confused.
Admist the chaos, there will be a relic. Pick it up if you wish.
You'll eventually reach an elevator shaft. Get on the ladder and get to the second floor.
Keep going up.
Eventually, you'll come across one of the more tougher enemies in this game, they also inhibt The Darkness, granting them heavy armor. Stunning and grabbing them is highly recommended.
Killing them rewards you roughly 3 times the skill points so do NOT forget to eat their hearts. Once the walkway collapses here, don't try to jump over. Simply walk the other way.
Once out of the building on the roof, you'll have to get back together with your fellow mafia friends.
It will mostly be an issue of jumping down in sequences.
Pick up the nearby relic on your way using Q.
Darkness enemies up ahead. Grab & Execute them.
There will be a large series of lights, having the enemies try to stun you again. Quickly sprint through all of them by holding (Shift).
After a few darkness enemies, you'll finally come across some ordinary ones wielding guns. Kill them and steal their hardware.
Eventually, you'll reach ground with your fellow mafiasanos who will drive you back to your now-invaded mansion.