Dark congress - Christopher Golden

Once upon a time, all the demonic races, together with the old gods, would send their ambassadors to the Dark Congress, which took place under a general truce once every hundred years. There they would discuss their different attitudes to humanity: some wanted to leave our world to its own devices, while others argued for conquest, and still others wanted peace with human beings. Mostly they would agree to live and let live. Centuries ago, the Congress and its pact was broken, and it has not been held since - until now. Called for the first time in over a thousand years, hundreds of supernatural creatures converge on Providence, Rhode Island. They need an independent Arbiter, someone to settle disputes between them; someone who understands their existence and shares it, yet who owes no loyalty to any faction amongst them. They choose Buffy Summers.