Arthur Allston served as a soldier in the Northern Army's Incarnate Soldier Unit during the Patrian Civil War. He was forcibly transformed into an Incarnate, a human-animal hybrid weapon, specifically taking the massive sauropod-like form known as Behemoth.

In this Behemoth state, Arthur reached nearly 50 meters in height. His body proportions were distorted compared to a typical sauropod, featuring an oversized torso and a relatively short neck that impeded proper posture. This immense size stressed his skeletal structure and joints, forcing him to move with excruciating slowness to prevent fatal injury. His Incarnate abilities granted tremendous physical strength capable of crushing structures underfoot, rapid regenerative healing that repaired damaged tissue within hours while toughening it against future assaults, and exceptionally thick skin highly resistant to conventional weapons like bullets and cannon fire.

Arthur spoke little and rarely showed emotion, maintaining a reserved demeanor. He followed military orders without complaint or visible personal desire, demonstrating unwavering loyalty throughout his service. After the war ended, he wandered eastward, deliberately avoiding populated areas to prevent harm to civilians despite his condition.

His final journey led him inexplicably towards a critical railway bridge. Though moving with deliberate slowness and showing no overt aggression toward settlements, his path endangered vital infrastructure. Local militias attacked, driving him into a pain-induced frenzy. Hank Henriette, his former captain, intervened to stop his advance. During their encounter, Arthur revealed no destructive intent; his sole purpose was reaching the ocean. Hank redirected him to a cliff overlooking the sea. Witnessing the ocean for the first time, Arthur experienced profound peace. A single tear welled in his human eye before he succumbed to his wounds and collapsed, dying with a contented expression.