Grimm & Kitty

A book depiciting a world where Death and his "cat" are on an adventure. Here death can not only be beaten, but killed, and once defeated, the killer may live for and extra 50 years, invincible to everything, until 50 years are up. Once a Reaper is killed another human is randomly chosen to be Death, for 100 years, they can die, and have many abilities, if they survive, they will become human again once their service is up.