Town of Ash
You will find yourself outside. Turn to the right and head down the street. Kill the hell spawn and defilers that appear until scene plays. Mop up any remaining bad guys and continue to the end of the street, kill the ravager here. Once the street is clear, enter the door on the left. At the end of the corridor, drop into the water and dive below, head over to the hole in the floor on the left and follow the underwater tunnel to the end.
Turn around and note the opening opposite the gap we just climbed up that is blocked by a half fallen door. If you jump to the left wall, wall run and then wall jump over to the broken door platform, you can actually leap over the broken door to find a small room behind it containing another chest.
Drop back into the water and swim back to the main pool. On the opposite side from where we entered, you will find a block you can wall run up to. Do so to reach a higher up ledge. Enter the door to the left and push the block down to the hallway below to create a shortcut. Backtrack all the way back to the street and this time around, enter the door on the opposite side.
Go down the stairs; death grip across the room and climb the stairs on the far side. Enter the door at the top and drop down the hole in front. As you do, the view will zoom out and you need to make your way across the room to the far right. As you reach your destination, a scene will play. When you regain control, continue to the end of the hallway and drop down into the pool of water below. Wall run up to the ledges on the far side and enter the door.
Kill the hell slaves that attack here and proceed to the far end of the room, noting the skull door on the left. Climb the stairs and before opening the door, turn around and wall run over to the ledge above the stairs to find a chest. Open this, then head back to the door and go on through.
On the left side of the balcony, wall run across to the opening on the far side of the room. Grab the stone block on the left and drag it over to the right. Plant it on the pressure plate. This will open the gate between the spikes to the east, so grab the chest there.
Once up top, follow the handholds all the way along until you drop down onto a platform. Enter the nearby door. Open the chest opposite the entrance, kill any hell slaves around and then cruise up to the doorway at the top of the stairs in the north east corner of the room. Use it to exit back on to the streets.
Town of Ash
Turn left and at the end of the road, drop down to the lower road below. Fight off the demons that appear until a scene plays to signify the end of the fight. Head over to where the concrete slab fell down during the scene and behind this you will find a set of stairs leading down. Descend all the way to the bottom.
Safe House Three
After a short scene in this area a boss fight will ensue.
Boss: The Warmaster
Charge: The warmaster will glow red and then charge at you from across the room, dash to avoid it.
Jumping Slam: The boss will jump at you and try to drop his axe on your head. Upon contacting the ground, this will also cause a circle of blades to emerge from the ground causing area of effect damage if you are too close.
Melee Swing: A standard slash. Blades will emerge from the ground below, doing extra damage if you are too close.
To defeat the Warmaster, the plan of attack is to, like many of Darksiders II's many bosses, dodge his attacks and slice him up a few times once he has just finished an attack. Once he reaches half health he will become enraged, regenerating all of his health.
At this point; he will also start moving a lot faster and his attacks will receive a small damage increase. Fortunately, the strategy remains essentially unchanged: it’s just a matter of learning his attack patterns and dropping a few cheap shots on him when you are able. Once he has taken enough damage, a scene will play to end the fight.
For finishing off the Warmaster, you will earn the skeleton key.