Buffy the vampire slayer : Stake Your Destiny, book 1: The Suicide King
Robert Joseph Levy et Joss Whedon

A rash of student suicides sweeps through Sunnydale, deeply unsettling the community. But when the newly-arrived grief counsellor also ends up killing himself, Buffy and the gang suspect that something supernatural is to blame. Then one of their own begins to show signs of debilitating despondency; and soon it becomes a race against time for the Slayer to defeat an ancient entity known as The Suicide King