The First Sinner is a hidden boss in Hollow Knight: Silksong, found in a place called The Slab. She is a Weaver condemned for "apostasy" due to discovering the origins of the Weavers as Pharlids. To reach her, Hornet must obtain the Apostate Key (found outside The Slab, in the Putrified Ducts) and break four silk runes locking her cell door. The fight starts when Hornet binds to a Weaver Statue inside the cell, which then reanimates as the First Sinner. The First Sinner is known for a fast-paced and challenging fight involving a combination of attacks such as summoning needles that shoot in various patterns, a healing bind ability that can be interrupted, and swift physical attacks like frenzied charges and aerial lunges. The fight also has two phases, with the second phase introducing more complex attacks like exploding runes (Rune Rage). If Hornet dies, the fight restarts outside the cell without penalizing the player or ending the game in Steel Soul mode. Upon defeating the First Sinner, Hornet can choose to accept the Weaver's power, granting a new ancestral skill called Rune Rage, which mimics an attack the First Sinner used during the fight. The lore suggests the First Sinner was the first Weaver to master weaving Silk into powerful runes, and a memory reveals her transformation from a Pharlid using a strand of Silk. This boss is highly regarded for its design and challenge, rewarding exploration and skill with both lore and gameplay progression. The fight's theme shares music with another boss called Widow, reinforcing their connection. In summary, the First Sinner is a key hidden boss in Silksong's Act Two, offering a meaningful fight experience, a new ancestral power, and significant story insights about the Weavers and Pharlids.