The Lost Sinner awaits the player below Sinner's Rise in a vast, dark chamber. Her soul description states that she eternally punishes herself for past sins, so one can assume she has entered this prison willingly as some kind of penance. The player must defeat her to embrace her great soul and progress the game.
As a boss she has rather straightforward attacks (although the tracking of them can get very tricky), consisting of greatsword swings and thrusts, and the occasional leaping attack. The real trouble with her fight comes from the unlit arena which can cause the player's lock-on to disengage and generally make keeping track of her difficult. This trouble can be bypassed however, if the player has defeated the Belfry Gargoyles first and obtained the Bastille Key. With it, the player can unlock the two doors next to her fog gate and light the room via torch.
The Lost Sinner, like Dark Souls 2's other three great souls, has direct connections to Dark Souls 1. Upon defeating her in NG+ she drops the Old Witch's Soul, the name of this soul combined with the fact that it can be used to craft either the Flame Weapon pyromancy or the Chaos Blade confirms that she possessed a piece of the Witch of Izalith's Lord Soul. Even more interesting, Sweet Shalquoir comments that she attempted to recreate the first flame and failed, much like the Witch of Izalith did before accidentally creating the Bed of Chaos as a result of her failure. Whether the Lost Sinner is some version of the Witch herself or an unnamed woman who discovered the Lord Soul and repeated the mistakes of her predecessor is ambiguous. Many fans of the series have noted that the bug in her eye during her opening cutscene looks highly similar to the Chaos Bug from the Bed of Chaos fight, but this could still mean the bug has simply found a new victim.