Apocalypse Zero

At the end of the 21st century, humans have made great advancements in technology and were able to create a giant energy source to power that technology. However, this new source of energy overloaded and turned into a more devastating force than nuclear weapons. The catastrophe wiped out the majority of human civilization and those who survived lived in terror of the anarchy that took over. Hagakure Kakugo comes from a secret family that has protected people for generations from powerful organizations and evil from taking over. They are only awoken in time of need and it has been decades since Kakugo was conscious. His body had been heavily modified by fusing it with metals to enhance both his offense and defense. He uses an armor called the Exoskull that attaches to him to greatly boost his already superhuman power. Thus begins a story about a young man and his armor’s journey through hell.

At the beginning of the Edo period (1603-1868), when people enjoyed a time of peace, Lord Tokugawa Tadanaga decides to hold a fighting tournament. In the past, matches were fought with wooden swords, but this time real swords will be used. The one-armed Fujiki Gennosuke and the blind Irako Seigen will fight each other in this tournament, even though both of them are disciples of Iwamoto Kogan — known as Japan's greatest swordsman. So begins the story of intertwining fates, conflict, and strange destinies. Note: It's SHOUNEN because it's published in Champion Red, a shounen magazine for late teens+early 20ies

With the power of the shogunate, Ieyasu, the supreme ruler of Japan, holds sway over the Sengoku Era. Those who refuse to follow his whims are doomed to be put to death by the god of war, Kibitsuhiko-no-Mikoto. But behold! Seven shadows are still rampant in this ruthless world. Are they men? Beasts? Petals? The wind? No, they are the seven blades of the Efu–the grudgeful ninja! Note: The Efu were the personal guards of the Emperor of Japan and his palace before the Feudal Era, following the establishment of Ritsuryō.

A series of natural disasters has reduced the world to rubble, with the survivors doing whatever they must to survive in a world gone mad. But one young boy, Kakugo, gifted with amazing martial arts and a superpowerful suit of armor by his late father, has been charged with making the world (or at least his school) a safer place. But his brother has a matching set of skills and equipment, and he's on a mission to bring peace to the world… by wiping out humanity!