Shishio was the successor to Kenshin Himura as the new top manslayer in the revolution until the government decided that they could not use him anymore because he had become too dangerous and they tried to kill him by setting him on fire. Instead of killing him, they burned his body so severely that he had to wrap bandages around his burns giving him his mummy look and raising his body temperature to such a degree that he could only fight for a certain amount of time and had to frequently cool his body temperature by taking baths. This led to him going against the government and trying to take over Japan.
Shishio had a pretty ambitious goal with trying to take over Japan, but he wasn’t just full of hot air, pun unintended. He had the group of people to do it with Sojiro, an expert swordsman with well hidden emotions as Shishio’s protege, Usui, the blind swordsman, who was an expert with a spear and turtle shell shield, the monk named Anji who was super strong and a master of the destructive punch known as the Futae No Kiwame, among many other powerful and unique henchmen, one of the best line ups for villains I have ever seen.
Even on his own, he was incredibly powerful. Once Kenshin and his comrades finally reached the boss fight against Shishio, Shishio managed to beat the ever loving shit out of Kenshin, Saito, Sano, Aoshi, the narrator and their uncle. Damn… You can screw with the mummy, but you won’t want to screw with this guy.