Riki-Oh is a fairly old Seinen Manga. It came out around the time when similar mangas(in terms of violence) like Jojo's Bizarre Adventure and Fist of The North Star were ranked supreme, so it was easily overshadowed. One look at the artwork and you can tell it's from the 80's, but it's great looking artwork none the less. This manga also has a lot of Buddhist overtones and borrows themes from the religion heavily. I started reading Riki-Oh a few days ago, and being a big fan of Seinen manga, I was immediately hooked.
Set in a dystopian alternative universe where global warming has devastated most of the earth and the world is suffering from an economical depression which led to increased crime. The story centers around Saiga Riki-Oh, blessed with inhuman strength and great intellect, after taking revenge against a yakuza who was responsible for the death of his girlfriend (in the manga a child friend of his who happened to use faulty drugs in a hospital after being run over by a car, turns out that the hospital was forced under threat by the yakuza to use their medicine), ends up in a maximum security prison owned by a private organization. The story follows Riki and his search for his little brother Saiga Nachi who bears a Swastika symbol on his right hand and also possess superhuman strength. Riki-Oh will encounter many deadly opponents with either superhuman strength or martial arts during his travel for avenging his mother and in search of his brother.