Sinner's Rise is the location of the fourth great soul boss and is accessed via the Lost Bastille. To find detailed instructions on how to reach it and what you can obtain on the way, read from the fourth paragraph onwards after the Ruin Sentinel boss fight in the Lost Bastille. For a very basic idea of where to go, teleport to the third bonfire of the Lost Bastille area (servant's quarters) and head through the door onto the battlements. Then just keep heading upwards whenever possible until you find a metal gate blocking a long bridge with a mist door at the far end. Pull the lever next to the metal gate to open it and sprint across the bridge to Sinner's Rise through the mist.
After walking through the mist be careful as the archers who were on the ledge above the mist door can drop down from the level above (they pull out their swords before dropping down), so be ready to engage any falling enemies. If you have ranged attacks, look down to the level below to pick off three archers below. When you are done, head through the door on the right side of this platform. Turn right as you exit to find a ladder that leads up to this area's solitary bonfire. At the top of the ladder, turn right into the small doorway and look to the right on the small ledge here to pick up a Large Soul of an Undead off a corpse. After lighting the bonfire, head back down the ladder and take the other path down the stairs.
Down the stairs, head past the door for a second to pick up a Human Effigy from a body at the bottom of the broken stairs. Return to the door and enter. Depending on your efforts with ranged attacks from above, there will be four archers minus the number you killed from above. The center of the room itself is a lift that descends down into the depths of the area. As you travel down, look for a corpse with an item on it on a small platform. You can step off the lift and land on the platform to retrieve the Lacerating Knife. Drop down from this platform to reach the lower waterlogged area.
From where the lift brings you, look behind it for the summon sign of Lucatiel if you are human currently. This watery area is home to a new enemy, water giants. They are large hulking enemies that have a decent amount of HP and hit for quite high damage. Most of their attacks rush forward as well and thanks to the slowing effect of the water they can be tough to deal with. Lightning spear works well against them (thanks to the water) and ranged attacks are a valid option. You can also use the lift to raise them into the air, dealing damage when they fall. Melee characters will most likely want to take advantage of the lift trick or lure them back to the lift room and engage them normally there (so the water doesn't slow you down).
The first giant is to your left as you enter the water from the lift room stairs. Then next pair are actually facing each other on opposite sides of the room. One is to the left of the central path, the other is to the right. Be careful when approaching the wall in the room on the right as the giant on the right can break through it and attack (this is a valid option to draw only one of the giants away, preventing you having to engage both at once). Behind where the first giant was standing is a body with Sublime Bone Dust on it. Head into the room on the opposite side (being careful of the wall here if you haven't killed the pair of giants yet. In the wall here is actually a hidden door that leads out to the water. Don't run forwards here as it will kill you, instead, look to the side to see a corpse holding a Bleed Stone and a Northern Ritual Band. Move along the narrow ledge while hugging the wall to reach it.
Back inside, head to where the second small area on the right side is (where the right side giant was) to find a body with a Large Soul of a Nameless Soldier on it. The remaining enemies in the area are exploding mummies (and with reduced running speed in the water, they are actually a threat). Used ranged attacks if you have them to pick them off in advance. They are in pairs, one on each side in the alcoves and cells all the way to the large gate at the end of the central path. Take the time to kill them all and grab the Pharro's Lockstone and Soul of a Proud Knight from the last cell on the left. Leave off activating the gate for now and travel back to the two sets of steps you went past (just after the mummies started appearing). At the top of both of these steps will be an exploding mummy resting against the wall (don't want them to drop down on you when you open the gate). If you have the Bastille Key from Belfry Luna (which you should if you have been following the guide), you will be able to open the locked cell up the left stairs and open the two locked doors to the left and right of the large gate. This bypasses the potential exploding mummy swarm when you open the gate. When you have finished killing all the exploding mummies, pull the switch next to the large gate and head on through.
Outside is a long thin bridge which you should cross to the other side of. Ignore the mist door for now and head into the narrow gap between the ramp and the wall to find a corpse holding a Radiant Life Gem. At the top of the two ramps here are oil trails that you can light that illuminate the boss fight room through the mist door. The benefit of this being you can see much better and can slow down the boss. You need the Bastille key from Belfry Luna to open those doors though. The room at the top of the right lamp also has a Smooth and Silky Stone on a corpse inside. When you are ready, enter the mist door to start the boss battle.
Boss: Lost Sinner
Personal Death Count: 0
Leaping Thrust: The lost sinner will retreat backwards towards the darker areas of the room and then jump towards the player and try to impale them. If the room is lit, this attack is easily avoidable (if she performs it at all - I've never seen it in a lit battle). In the dark it is much harder to avoid as the auto lock on doesn't work (making it harder to keep track of her movements). If she disappears, back up (preferably with your back to a wall and put up your shield. If you are a ranged character, sprint until she lands (making her miss) and then attack.
Multi Slash: She will quickly move her sword behind her back (in one flowing motion, no delay like her other attacks) and perform a large slash attack. This can then be followed up by a downwards slam or a slash. Sometimes she will perform a slash, slash and slam attack as well. Assume the worst and roll backwards twice to keep safe.
Slash: The lost sinner will move her sword behind her back, then step forwards a reasonable distance while slashing in a large arc. Rolling backwards will get you out of danger, or a well timed dodge to the side.
Sprinting Lunge: She will turn her upper body ninety degrees with her sword behind her and then sprint towards the player, lunging with the blade straight ahead when she nears them. Roll to the side to avoid this attack.
There are two main pieces of advice to make this boss as easy as possible. First, have the bastille key (from defeating the belfry gargoyles in belfry luna) and light the torches up the ramps just outside the boss room. This slows down the lost sinner and makes it far easier to track her movements. Without the torches, she is a pain to deal with despite her basic repertoire of attacks. This is due to her speed and the tracking of some of her attacks. The second recommendation is to bring NPC Lucatiel to this fight (her summon sign is next to the lift (lower level) that brought you down to the lower area. If you are a magic user, you should be on easy street here. Load up your most powerful spells and go to town on her (see the video). After learning the timing of her attacks and when to dodge you should have little problem defeating her in the lit up room. If the room is not lit, you will need to hone those dodges to a fine art (due to increased speed) and watch out for her leap attacks as she can sometimes enter another attack immediately afterwards.
Melee characters will need to stay just out of her reach and bait her slash attacks (slash or multi slash). This will allow you to follow up her chain attack with your own attacks or block her single slash with a shield and then attack. In new game plus, two pyromancers with a large variety of pyromancies join the fight as well when the lost sinner drops below 60% health. Use your summon ally and / or a shield with high physical and fire defense to protect yourself. Players may also want to invest in the Redeye Ring that draws enemy aggro solely to the player. This means your summoned ally will be ignored and can deal damage while you dodge and roll away. After enough perseverance or a few good spells she should go down. But the biggest thing is lighting those torches, they can make or break the fight.
After the boss battle is over, look for a door on the other end of the boss room from where you entered. Descend the steps and look to the left in the small room at the bottom for a chest containing a Fragrant Branch of Yore and an Elizabeth Mushroom. Walk into the large room beyond and light the fourth Primal Bonfire and teleport back to Majula.