Sword of the Dark Ones

Sword of the Dark Ones is a manga series by Tsukasa Kotobuki, and it is based on a story by Kentaro Yasui. This series is also known in Japan by the title Ragnarok (ラグナロク). In North America the story is published by CMX.

Plot
The story takes place in the fictional Ragnarok continent, which includes the Asgard Empire. People live their lives in constant fear as the land is overrun by flesh-eating monsters known as "The Dark Ones". They roam the land and hunt from the darkness killing indiscriminately. To combat this threat, a guild of mercenaries was formed. One man in particular has built a reputation as an especially fierce monster-killer, a man known as Leroy Schwartz, or the Black Lightning. He is a man with a mission to accomplish and a vow to fulfill, his only companionship lying in his sentient long-sword Ragnarok.