Monster Strike

Homura Ren goes back to his hometown after living many years away, with his family. He barely remembers the city, and forgot all about his friends. Right in the first day, his cell phone is damaged and send to a Repair shop. When he gets it back, Ren find a different app installed. It's Monster Strike. Suddenly, a huge monster shows up, attacking him, and a voice in his cell phone asks to be release in exchange to save him.What truly is Monster Strike?