The Harlequin (Anita Blake Vampire Hunter #15) by Laurell K. Hamilton

They will play with you, then destroy us...
They are what we fear in the dark.

The first warning is unexpected, calculated.
The second warning is a gift: a plain white mask, carefully wrapped. But white is good -- white means we are only being watched.

It seem the power that connects me, Anita Blake, with Jean-Claude, Vampire Master of the City, and Richard, leader of the werewolves, is attracting very unwelcome attention - from creatures so feared no vampire will willingly spreak their name. They are known as the Harlequin, and they have the authority to pass judgement upon me.

It is forbidden to speak of the Harlequin unless you've been contacted. And to be contacted is to face a sentence of death.