"Sekiro Side Story: Hanbei the Undying" is a manga adaptation that expands on the lore of the "Sekiro: Shadows Die Twice" video game, focusing on the character Hanbei the Undying. The story is set in the war-torn Sengoku period of Japan and explores Hanbei's backstory, his curse of immortality, and his connection to the broader world of "Sekiro."

The narrative begins with Hanbei as a skilled but mortal samurai serving under a warlord. After a brutal battle, Hanbei is mortally wounded and left for dead. He is discovered by a mysterious monk who offers him a chance to live again through a forbidden ritual. Hanbei accepts, unaware of the consequences, and becomes immortal. However, his immortality comes at a cost: he cannot die, no matter how grievous his injuries, and he is haunted by the memories of his past life and the lives he has taken.

As the story progresses, Hanbei wanders the land, seeking purpose and redemption. He encounters various individuals, including warriors, villagers, and supernatural entities, each with their own struggles and ties to the ongoing war. Hanbei's interactions with these characters reveal the moral complexities of his immortality. While he uses his unending life to protect the innocent and fight against evil, he is also tormented by the knowledge that he cannot truly die and escape his guilt.

A significant portion of the story delves into Hanbei's relationship with the Divine Realm, a mystical dimension tied to the source of his immortality. Through flashbacks and encounters with other immortals, it becomes clear that Hanbei's curse is part of a larger, darker scheme orchestrated by forces beyond human understanding. The manga explores themes of sacrifice, the burden of eternal life, and the cyclical nature of violence.

The climax of the story sees Hanbei confronting the monk who granted him immortality, as well as the malevolent entities that seek to exploit his curse for their own ends. This confrontation forces Hanbei to make a choice: continue living as a tool of war or find a way to break the cycle of suffering, even if it means sacrificing his own existence.

Throughout the manga, Hanbei's character is portrayed as a tragic yet noble figure, driven by a desire to atone for his past and protect others. The supporting cast, including the monk and other immortals, adds depth to the narrative, highlighting the moral ambiguities of immortality and the consequences of tampering with life and death. The story remains tightly connected to the themes and world-building of the "Sekiro" universe, offering fans a deeper understanding of its lore while standing as a compelling tale on its own.