I don't know when a mobile phone software called "Desperate Mail" became popular among middle school students. This software will send short animated short videos to users about how the user "died" from time to time through photos, videos, etc. That are usually uploaded. Because excitement and fun are popular. At the same time, a creature appeared in the world. They have special abilities. They feed on humans. They can even transform into humans. No one knows their origins. Humans call them "demons". The software "Death Mail" actually has another use: if the user dies due to being attacked by a "demon", the system will ask if he needs to be resurrected. If he chooses to be resurrected, he will become the "summoner". The "Summoner" can summon the "Guardian" to fight against the "Demon". However, the number of software with this function is limited in the world. Only one in every 100 models. What exactly will the "summoners" do with their abilities? Who is the designer? Where did the "demon" come from?