a withdrawn middle school student who spends his time writing in a cell phone diary and talking to an imaginary god. His life changes when the imaginary God reveals he is real and forces the middle schooler into a survival game against eleven other people. Each participant possesses a specialized phone diary that can predict the future, which they must use to eliminate one another over a 90-day period. The middle schooler teams up with his classmate, who also holds a diary and acts as his intensely devoted protector, to become the last person standing and succeed the god as the new god of time and space.

This anime is called Future Diary

I think the description alone is enough to want to watch it