Fate/stay night: Heaven’s Feel II. Lost Butterfly

Sakura is revealed as Rider's true Master when she calls upon her during Shinji's kidnapping. While Shinji had been controlling Rider with stolen command seals, Rider was always bound to Sakura as her original Master. When Sakura desperately pleads for help, Rider responds to her true summoning, abandoning Shinji's control and revealing Sakura's hidden role in the Holy Grail War.

Earlier scenes show Rider acting strangely loyal to Sakura despite Shinji's commands.

Crest Worms are parasitic creatures placed in Sakura's body by Zōken Matou when she was adopted into the family. They feed on her mana by constantly stimulating her sexually, which weakens her physically and mentally over time. As the Worms awaken, they amplify her desires and emotional instability, gradually corrupting her from within.

Sakura's erratic behavior and emotional fragility hint at the Worms' influence throughout the story.

Archer transplants his own arm to Shirou after the shadow severs Shirou's left arm during battle. This act saves Shirou's life, but it also transfers Archer's abilities and memories into Shirou's body. The arm is sealed with holy cloth to contain its power, allowing Shirou to survive with strength he never trained to wield.

Archer's earlier interest in Shirou's ideals set up his willingness to make this sacrifice.

Yes, Rin and Sakura are blood sisters. Sakura was born to the Tohsaka family but was entrusted to the Matou family as a child. This secret explains why both girls possess magical potential and why Rin feels conflicted about Sakura's fate throughout the war.

Their similar magical aptitude and Rin’s unexplained guilt hint at their shared lineage.

The shadow is a fragment of the Holy Grail that Zōken Matou implanted inside Sakura ten years ago. It manifests as a dark, amorphous entity that can consume anything it touches. As Sakura's mental state deteriorates from the Crest Worms, the shadow gains power and eventually breaks free, threatening to destroy everything around it.

Zōken's interest in Sakura and the dark energy surrounding her foreshadow this corruption.

Sakura walks through the city at night in a trance and kills several people because the Crest Worms and the shadow inside her are corrupting her mind. She has no memory or control over these actions, the darkness within her takes over while she sleeps, turning her into a dangerous threat she never intends to be.

Her increasing emotional instability and the Worms' awakening set the stage for this breakdown.

Shinji attempts to rape Sakura at the Matou estate, blaming her for his failures as a magus. In a panicked fit of desperate violence, Sakura kills Shinji. Immediately after, the shadow fuses with her body completely, corrupting her into the Holy Grail vessel and ending any hope of saving her.

Shinji's abusive behavior and Sakura's growing power hinted at this violent confrontation.

Gilgamesh attacks Sakura while she is sleepwalking, attempting to destroy her. However, the shadow inside Sakura is far more powerful than expected, it engulfs and consumes the King of Heroes, absorbing his power and strengthening itself further. This moment proves the shadow has become nearly unstoppable.

Gilgamesh's arrogance and the shadow's previously shown consuming ability set up this confrontation.

Illya reveals she is Kiritsugu Emiya's biological daughter, making her Shirou's adoptive sister. Kiritsugu was the winner of the previous Holy Grail War, and he left Illya with the Einzbern family while adopting Shirou. This connection adds tragic depth to their relationship and explains Illya's interest in Shirou's survival.

Illya's unusual attachment to Shirou and her knowledge of Kiritsugu hinted at their family connection.

Zōken Matou implanted Crest Worms into Sakura's body when she was adopted into the Matou family as a child. He also placed a fragment of the Holy Grail inside her, creating the shadow that eventually corrupts her. This was done to make her a powerful vessel for the Grail's energy, sacrificing her mental and physical wellbeing for magical power.

Zōken's cold interest in Sakura and the mysterious Matou family history foreshadowed this dark experiment.