The Mark of Mortem

After watching a man come in to her class and kill her math teacher, Corinne Dorsey finds out she is the only one who can see him. After he takes her with him, she finds out that he is an Deliverer of Death, immortal beings who are sent off to find people when it is their time to die. Normal humans cannot see the Deliverers, but something is going on in After Life, and Corrine is the center of it.