Getting to Sinner's Rise
After defeating the Ruin Sentinel, you will be led to the Servant's Quarters bonfire, it is from here that you can go to either Sinner's Rise or Belfry Luna.
To get to Sinner's Rise, head up the staircase opposite the bonfire. Just outside the door is a scythe guard. The guards are slightly different now as they possess fire spells, but these are easily avoided thanks to their long cast time. There's a human effigy behind the barrels on the edge of the bridge. At the other end is a tower, head through the second door and kill the other scythe guard.
Head along the path and onto the rooftops. Grab the Flame Butterfly and jump to the opposite roof, grabbing the Large Soul of a Nameless Soldier. Head up the ladder and pull the lever to open the gate. The mummies in this hallway are much like the ones you faced in The Lost Bastille, the ones with the glowing chests do greater damage and causes petrify, so try and quickly deal with these guys.
In the next room are mummies hiding in the pottery, so slice away at everything in the room to be extra safe. Head up the stairs and kill the sitting mummies before they have a chance to wake. Open the door on this second floor and grab the Skeptic's Spice, you can also drop down into the cell to grab the Bonfire Ascetic, but the only way out is to drop down once more onto the small platform, and there's a strong chance you'll die, so it's probably worth avoiding for now.
Back on the second floor, head out of the door and sprint across the bridge to avoid fire from the archers, and you've made it to Sinner's Rise.
Once you're through the mist, head through the second door on the right and up the ladder to the right again and ignite the bonfire. Next to the bonfire are three crossbowmen, the respawn every time you teleport back to the bonfire when you die, and will start firing on you immediately, so be careful. Take these guys out and grab the Large Soul of the Lost Undead from the ledge just inside the door by the ladder.
Head back down the ladder and proceed down the stairs, killing the crossbowman at the bottom. Just past the doorway is a Human Effigy, grab it before dealing with the enemies inside. Through the doorway are three crossbowman stood around a lift in the centre of the room. Your best bet is to run past them one at a time to make them draw their sword, then head back to the stairs to avoid fire from the other two, killing them one at a time.
Once all the the silver knights are dead, take the elevator. You want to jump off the lift once you see the wooden platform to grab the Lacerating Knife. Drop down once again and send the lift back up, this is an important tactic to deal with the very tough ogres in the swamp you're about to enter.
Head down the stairs and into the water, turn left and taunt the first ogre. These guys have over 1000 health and attacks from even the strongest of tanks can do as little as 50 damage. Your best bet is to lead them back to the area with the lift and send them into the pit. The ogre should walk straight into the abyss, but if it doesn't, taunt it to attack and it should soon fall.
Head back down, turn left and grab the Sublime Bone Dust, before turning right and taking on the next ogre found on the opposite side, using the same tactic. From the elevator, head down the the stairs and turn right, there's an illusory wall in the first room to the right (for reference, the ogre was in the second room), press the action button to reveal it. Head through the wall and turn left to grab the Northern Ritual Band and Bleed Stone. DO NOT WALK OUT INTO THE LAKE, this is an insta-death. Just go back into the sewer and proceed forward.
There's a third ogre to the left, lead him back to the lift and use the same tactic. Lucatiel's summon sign also appears in this room, providing you are a human, and she can be very useful in the boss fight against the Lost Sinner.
There are mummies littered all through the rest of the sewer, some are hidden as they're sat below the water level, so be careful when you walk into cells. You have to kill all the mummies before pulling the lever at the end of the room, as they'll all wake up when you pull it. Any gate is locked at this point, so just walk through open or broken doors. The top-left room houses a Soul of a Proud Knight and Pharros' Lockstone, too.
Once all the mummies are down, pull the lever at the end of the walkway and head along the walkway. There are two spiral staircases leading above the mist door, below the stairs to the left is a Radiant Lifegem.
Proceed through the mist to face the Lost Sinner. If you've beaten the Belfry Gargoyles before entering Sinner's Rise, you can use the Bastille Key to open the doors at the top of the spiral stairs. Use a torch to light the oil to make sure the arena stays bright during the match, making it easier to lock onto the boss.
How to beat the Lost Sinner
The Lost Sinner is much easier to beat if you're a tank character. Also, having Lucatiel along for the fight helps spread the Sinner's attention between the two of you. If you haven't fought the Belfry Gargoyles first, the room will be pitch black.
Try and keep your back to a wall to prevent the Sinner from getting past your shield. Once it's in sight, circle and look to attack following its attempt to hit you with single or double strikes. Having high stamina and strong defence will make this drastically easier.
The Sinner uses a three hit combo, as well as a stab attack. Counter with single strikes to help your stamina recover. This is pretty much a battle of attrition; the Sinner gives you plenty of opportunities to attack, but none of them do substantial damage. Keep plugging away and eventually the sinner will fall.
You will receive the Soul of the Lost Sinner, which can either be absorbed to gain 25000 souls or given to a weaponsmith found later on in the game.
Head through the door at the opposite end of the room (use a torch if you're struggling to find it) and ignite the bonfire to return to Majula.