The Last Zanpakutō

The door to the present world opens for no reason, a large number of demon spirits invade Heian Jing, and the Heian world is on the verge of collapse. Can they find the Zanpakuto lost thousands of years ago and save Kyoto from the crisis? How was Soul Society created? Where did Onmyoji come from? All the answers have to be found from a disaster that will happen one thousand three hundred years later...