Lost Sinner

The Lost Sinner is a boss in Dark Souls II.

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

Description
A possessed corpse or undead slightly larger than the player, wearing an iron mask and a wrist shackle. She wields a large greatsword, and uses it with great speed and efficiency.

Lore
The Lost Sinner is a forgotten entity, presumably undead, who imprisons 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 she imprisons herself, or if the attempt was a means of redemption. It might also be speculated that Nashandra, the Queen of Drangleic, stopped her.

Summoning
Lucatiel of Mirrah can be summoned for this battle.

Strategy
This is a challenging boss, and her difficulty is only made worse by the fact that the room she is fought in is normally very dark. This obscures her movements and prevents the player from locking on to her unless at close range. However, the room can be lit up. Defeating the Belfrey Gargoyles and getting the Bastille Key allows you to unlock the two doors on each side of the boss's fog door, and light the oil inside with a torch which illuminates the boss's room.

Even with the room lit, being careful is essential. Use your best shield and be ready to block her attacks, as she can easily stunlock you and kill you. Still, her attacks do high stamina damage when blocked, so be careful. Staying close and circling her, then attacking when she does her thrusting, or downward smash, attack is a decent tactic, but the boss jumps around the arena often so it may be difficult to stay consistently near.

Be careful when healing or using items as she is highly aggressive and will not stop attacking for very long. She can be parried, but quickly jumps away before any sort of follow-up attack can be performed. Using magic against her is inadvisable, but possible if you are confident in your dodge timings.

On New Game + and beyond, two black phantom pyromancers drop into the room when The Lost Sinner drops to around 50-60% health. They will continually run around the room and cast pyromancies at the player at the same time as The Lost Sinner gives pursuit. They frequently dodge magic attacks and sometimes dodge melee attacks. 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 a high damage dealing potential, it is possible to defeat The Lost Sinner while both pyromancers are active. However, dodging all three enemies can be very challenging.

After defeating The Lost Sinner before defeating the pyromancers, it is possible that the pyromancers will turn neutral and stop attacking the player.

Attacks
Horizontal Slash Combo: Her most common attack, often done twice in a row. It can be blocked, but she often will follow this move up with a downward sword slam that will almost always break your guard if you don't dodge.

Thrusting Attack: A powerful stabbing attack with the greatsword. Stay close and circle around her. If she misses, it gives a good chance for the player to counterattack.

Vertical Slice: 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. If she overshoots the player, she may turn around and perform a stabbing attack.

Parry: She will block with her greatsword and if the player attacks her, will parry and riposte them. This can cause massive damage and will likely kill the player. As she remains stationary when blocking, it is a good time to hit her with ranged attacks.