Fate/stay night: Heaven’s Feel I. Presage Flower

Shirou became a Master after witnessing a battle between two Servants on a night when he had quit the archery club due to injury. Saber appeared and called him Master, revealing he had already been chosen as a Master. Rin Tohsaka later confirmed this by taking him to Kirei Kotomine, where he learned he was bound to Saber through the Holy Grail War system.

Earlier that same night, Shirou witnessed a clash between two mysterious warriors, foreshadowing his entry into the war.

The Shadow is a mysterious, deadly entity that targets both ordinary citizens and Servants throughout Fuyuki City. It appears as a dark consuming force that devours its victims, including Servants like Lancer and Caster. The Shadow is controlled by Zouken Matou, who uses it as a weapon in the Holy Grail War.

The illness spike affecting citizens is caused by the Shadow's presence and activities.

Zouken Matou uses Crest Worms to sustain his life force by feeding on others. He revealed this by controlling Caster's corpse like a puppet and later demonstrating by devouring a woman in an alley. The Crest Worms are parasites that allow Zouken to survive beyond his natural lifespan, and he uses them to manipulate and control other Masters and Servants.

Zouken's earlier humiliation of Shinji hints at his ruthless nature and willingness to use others as tools.

During the climactic confrontation at Ryuudou Temple, Shirou's command spells vanished when the Shadow captured Saber. The command spells are tied to the Master's connection to their Servant, and when Saber was taken by the Shadow, that bond was severed, causing Shirou to lose his authority over Saber.

The Shadow's ability to consume Servants was established when it devoured Lancer and Caster earlier.

Shirou persuades Sakura to stay at the Emiya residence to keep her safe from the dangers plaguing the city. With the Shadow targeting people throughout Fuyuki and the Holy Grail War escalating, Shirou wanted Sakura protected under Saber's guard. Sakura had expressed concern for Shirou's safety and their bond grew stronger through this arrangement.

Shinji's denial of Sakura's involvement in the war earlier hints at the danger she faces as a Matou family member.

Illyasviel von Einzbern, a young girl who is also a Master in the war, attacked Saber alongside her Servant Rider's master Berserker. The attack occurred on a street after Rin revealed the truth about the Holy Grail War to Shirou. Illyasviel withdrew after witnessing Shirou sacrifice himself to protect Saber, proving his resolve to fight.

Illyasviel's identity as a Master was established through her presence and the combat capabilities she displayed.

Lancer was ordered by his Master Kirei Kotomine to track down the new Assassin. After a long chase through the city outskirts, Assassin lured Lancer into a lake where the Shadow devoured him. Lancer's death marked another harsh blow in the war and demonstrated the Shadow's deadly threat to even powerful Servants.

Kirei's warning about the Shadow being a threat to the entire city foreshadows Lancer's fate.

Rin Tohsaka and Archer broke into the Matou residence basement to investigate the source of the mysterious illnesses affecting Fuyuki's citizens. They discovered the basement was infested with Crest Worms, revealing the scale of Zouken's manipulation and tying the darker secrets of the Matou lineage to the Grail War's conspiracy.

The earlier illness spike and Zouken's control of Caster pointed to his involvement in dark machinations.

When Rider's identity as Shinji's Servant was exposed, Shinji's grandfather Zouken ridiculed him for failing. Shinji then denied Sakura's involvement in the war as he and Zouken departed. This denial suggests Shinji was trying to protect Sakura from the dangerous politics of the Matou family, or perhaps trying to hide her connection to their schemes.

The revelation of Crest Worms in the Matou basement later connects Sakura to this dark family legacy.

Rider arrived at the last moment during the final battle at Ryuudou Temple to drive away the attackers, granting Shirou and Saber a narrow window to retreat. Although Rider was summoned by Shinji Matou, she chose to help Shirou, suggesting she recognized his noble character or had her own reasons for opposing Zouken's scheme.

Rider's defeat by Saber earlier in the war did not destroy her loyalty, and her intervention suggests deeper motivations.

Shirou became Saber's Master when he was chosen by the Holy Grail War system. Saber protected him from danger and called him Master from their first encounter. Their bond deepened when Shirou sacrificed his own safety to shield Saber from Illyasviel's attack, proving his resolve to fight alongside her.

Saber appeared to protect Shirou on the night he witnessed the warrior clash, establishing their early connection.

Kirei Kotomine revealed himself as Lancer's Master and warned Shirou that the Shadow posed a threat to the entire city. However, Shirou refused to trust him or join forces. Kirei's warning seemed genuine, but his true motivations remained ambiguous, adding to the moral complexity of the Holy Grail War.

Kirei's status as a priest and Lancer's Master creates intrigue around his true intentions.