Lost Sinner

The Lost Sinner or Forgotten Sinner in the original text is a boss in Dark Souls II.

Location
The Lost Sinner is located in Sinner's Rise.

Description
An unnaturally tall undead woman clad in a tattered straight jacket and skirt, wearing an iron mask and her hands bound in cuffs.

Lore
The Lost Sinner is a forgotten entity, presumably undead, who imprisoned herself for sins committed long ago.

According to Shalquoir she tried to kindle the First Flame, but it's impossible to know whether this failure is the reason for what she did, or if the attempt was a means of redemption. It might also be that Nashandra, the Queen of Drangleic, stopped her.

Summoning
Lucatiel of Mirrah can be summoned for this battle. Her summon sign can be found after getting off the elevator, just before the flooded area.

Strategy
The Lost Sinner is a challenging boss and her difficulty is only made worse by the darkness of the boss room. This obscures her movements and prevents the player from locking on to her unless she is at close range. However, defeating the Belfry Gargoyles and getting the Bastille Key allows you to unlock the two doors up the stairs on each side of the fog door and light the oil inside with a torch which illuminates the boss' room.

Even with the room lit, being patient and careful is essential. A shield with good stability will lessen her chance to easily stunlock and likely kill the player, as her attacks do high stamina damage when blocked. Players should only attack when enough stamina remains to roll/block afterwards.

Staying close and circling her then attacking while she recovers from her attacks is a good tactic. If she jumps away, you can close in on her or let her come to you. She will usually move in with a dashing attack or her jumping attack.

Be careful when healing or using items as she is highly aggressive and will not stop attacking for very long. The Lost Sinner can be parried, but she will quickly jump away before any sort of follow-up can be performed. Using magic against her won't be easy, but possible if you are confident in your dodging skills.

On NG+ and beyond, two black phantom pyromancers drop from cages on either side of the room when The Lost Sinner falls to around 50-60% health. These pyromancers use a variety of attacks to kill the player such as Flame Swathe, Forbidden Sun, Chaos Storm (Dark Souls II) and Great Chaos Fireball (Dark Souls II). They will continually run around the room and cast pyromancies at the player, and are hard to hit as they frequently dodge both magic and melee. One strategy is to summon Lucatiel of Mirrah and attempt to damage The Lost Sinner enough that the pyromancers appear so that you can quickly take them down before Lucatiel is defeated, as the boss may focus on Lucatiel if she is alive. If a player has high damage dealing potential, it is possible to defeat The Lost Sinner while both pyromancers are active. However, dodging all three enemies will be challenging.

If The Lost Sinner is defeated before the pyromancers have been dispatched, it is possible that the pyromancers will turn neutral and stop attacking the player.

One of the best strategies to defeat this boss is to use the Redeye Ring that you can purchase in Majula from Sweet Shalquoir. Use a shield with 95 - 100% physical damage reduction and good stability, such as the Drangleic Shield. Hold it in two hands, then summon Lucatiel of Mirrah or another player as a phantom and then block the entirety of the fight. Draw the Lost Sinner around in a circle, blocking her attacks and sidestepping her overhead swings/jump attacks. Lucatiel will chase the boss and attack it in the back, and avoid being damaged almost entirely. This is also the recommended strategy if you wish for Lucatiel to survive this fight in order to progress in her sidequest. The same method can be used to defeat a number of bosses, including the Ruin Sentinels. This strategy isn't as good in NG+ and up, but can still be effective with some heavy hitting phantoms to help deal with the pyromancers quickly, or if you have a Gyrm Greatshield or a similar shield with high fire resistance.

Attacks
Horizontal Slash Combo: Many versions of her horizontal slash combo. Her most common attack, often done twice in a row. It can be blocked. One where she performs a horizontal slash, then followed up by an overhead attack, large stamina drain. Another combo she does is when she slashes twice in a row then followed by a thrust strike.

Quick Thrust: A powerful stabbing attack that is easily dodged by rolling under. Staying close and circling around her can give a good chance for the player to counterattack.

Uppercut Strike: An upward slice, sometimes followed by a downward slice. Dodging is the best option.

Jumping Attack: Jumps out of view and attempts to hit the player with a falling attack, plunging her blade into the ground, sometimes if the player is near her though the attack missed, will follow up with a horizontal swipe. If she overshoots the player, she may turn around and perform a stabbing attack.

Parry: She will go into a stance where she holds her weapons hilt against her face, if attacked during this stance she will block with her greatsword and perfrom a parry and riposte them. This can cause massive damage and will likely kill the player.

Quick Swipe: Swipes her blade quickly, sometimes she follows it up with a back step.

Charge: If the player has created distance between her, she will perform a charging thrust attack.

Backstab: If the player turns their back to Lost Sinner, she'll backstab them dealing not as much damage as Parry but will deal a large amount of damage; footnote: Horizontal Slash Combo deals more damage if all hits land.