Part II of Sword Gai: The Animation.

When the gods would not answer in humanity's desperate hour of need, it turned to a demon instead. The supposed savior came wielding the sword Zsoltgewinn, but its uncontrollable lust for blood led it to kill those who summoned it too. Although the sword was sealed away long ago, it has been uncovered by the Shoshidai, an organization that collects such cursed artifacts. However, Zsoltgewinn proves to be too strong to be tamed by humans when its corruptive power influences the administrator, Takuma Miura, to flee with it in his grasp. At the same time, Gai Ogata's family is torn apart due to the possession of another demonic sword, Shiryu, leading his father to be murdered and his mother to hang herself shortly after giving birth to him. Abandoned in the forest clutching the blade, he is discovered by the blacksmith Amon. Unnaturally transfixed by the sword, Gai works tirelessly for years to hone his smithing skills. However, when an accident costs him his arm, he gains a new one—in the form of a reforged Shiryu. Now having a cursed sword for an arm, Gai must learn to control its violent urges. All the while, Zsoltgewinn continues its rampage, leaving a path of blood in its wake. [Written by MAL Rewrite]

Long ago, a goddess descended from Heaven and blessed the desolate land of Gatepia. As a result, gigantic gates appeared, leading to dungeons abundant in "kirakuri," crystals containing the energy needed for the foundation of the world. This led to the formation of various companies of adventurers who would harvest kirakuri from the dungeons. Following his father's disappearance inside one of the biggest gates in Gatepia, Minato is urged by his childhood friend Yutoria to become the president of his father's treasure-hunting company—the Kibou Company. He reluctantly agrees and meets with the other employees: the priest Makoto, the soldier Akari, and the accountant Guide. Thus, Makoto begins his tenure as president. As he and his comrades strive to fulfill various missions and other assorted tasks in order to keep their small company alive, they will uncover the mystery behind their former leader's sudden departure.

"Mine has been a life of much shame."

Tokyo, 2036 (Showa year 111): a revolution in medical treatment has conquered death...
By means of internal nanomachines and the "S.H.E.L.L." system whose network controls them, human beings suffer no diseases, require no treatment for injuries, and are guaranteed a 120-year lifespan, free from illness. Yet this consummate social system warps the Japanese nation in a host of ways: unresolved economic disparities, ethical decadence resulting from deathlessness, grave environmental pollution, and the "Human Lost" phenomenon, in which people themselves, disconnected from the S.H.E.L.L. network, become malformed. Japan teeters wildly between two potential futures: civilization's restoration or its destruction.

Atmospheric pollution suffuses "Route 16" in the Outside—the area outside the Route 16 beltway. Youzou Oba, who lives an idle, drug-saturated life, joins Masao Horiki, a mysterious man who associates with the drag-racing gangs, on an incursion Inside—the area within the Route 7 loop where the privileged class lives—only to be embroiled in a violent struggle. When he encounters a malformed sufferer of the Human Lost phenomenon, a "Lost," Youzou's life is saved by Yoshiko Hiiragi, a girl of mysterious abilities who belongs to the anti-Lost agency H.I.L.A.M., and he discovers that he himself also possesses extraordinary powers...

Degradation and death. Life and hope. Buffeted by fate, a man tears himself apart and cries out. Rage. Sorrow. Pathos. Consumed by despair and bitter tears, Youzou Oba is himself transformed into a demon. A human lost... or a human who can find himself? 

(Source: Official Site)