50,000 years ago, Hell was destroyed because of too much Foul from the resentment of the souls that later turned to Soul Eaters and devoured everything in their wake, leaving nothing of Hell. The gods above in Heaven, in order to take care of the newly-died souls, created the Afterlife Department comprised of grim reapers.
Huo Ling was delivering food that day when he met Yi Bing and the guy gave him a warning. When he arrived to the customer's unit, he was killed.
Huo Ling: you set up a death flag for me!
Yi Bing: I'm a grim reaper. I'm the flag myself. Let me help you solve the mystery of your death.
Huo Ling: your sister!
Alternative title: The Different Ways I Died at 23