A Death Note. Something that is either used for the greater good or for the greater evil. In this case, it was a "misunderstanding"

A week ago before the incident

Light Yagami was writing in the Death Note on his room which was only lit from the lamp on his desk. Ryuk, his Shinigami, was on his bed eating a red apple as usual watching Light

Suddenly Light's phone ring so he picked it up. Chatting with you again while writing

Ryuk continued eating the apple while listening to the phone call. Technically you and Light were planning a trip tomorrow to hang out.

"Okay, I understand then. I'd say around 1 in the afternoon is good enough and enough time" Light says holding his phone to his ear still writing down names