Midgard no Shugosha - Re-Birth of Norse Mythology

Soah is a lively human boy who dreams of becoming strong enough to defeat the Jotunn. He is also inspired by Thor, the heroic God of Thunder, who has patrolled around Midgard in order to eliminate all the Jotunns, protecting powerless humans, and Soah's friend. One day, after Thor returned to Soah's village for a short stay, Soah got captured by the Jotunn while he's training. Despite of his hesitation to attack the Jotunn because of the friendship they shared, Thor finally destroyed the Jotunn, but killing Soah as the result as well. Seeing Soah's determination, Thor bestowed all his Meging into reviving Soah, sacrificing himself. Several years passed by since the incident, Soah now is traveling out of his village along with three Meging weapons Thor left behind, in order to carry out Thor's will and protect Midgard from the Jotunns.

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From MangaHelpers:

In this story of Norse mythology, humanity's world of Midgard is caught up in a war between two of two powerful divine races, the Jotunn and the Aesir. Humans can't stand against them, due to the godly 'Megin' power of the divine races. The Jotunn see conquering Midgard as the first step toward attacking the home of the Aesir, and they slaughter many humans along the way. The Aesir step in to rid Midgard of the Jotunn and protect the defenseless humans. One of these Aesir, Thor, made his home base in a certain grateful human town.

In this town lives a young boy named Soah, who lost his parents in a Jotunn attack. He idolizes Thor and dreams of one day becoming strong enough to fight Jotunn and protect people. Thor enjoys Soah's childish enthusiasm, and the two of them share a warm friendship. One day, a Jotunn invading the town uses Soah as a human shield. Thor protects the town, but at the cost of the life of his young friend. Unwilling to let the strong-hearted boy die, Thor bestows his Megin upon Soah and vanishes. After much training, the now-powerful Soah begins to realize his dream, fighting against the Jotunn in Thor's place! [tethysdust]