The Keeper and the Rune Stone (The Black Ledge Series)

An unholy ritual, a ticking clock. Four children discover there are things that go bump in the night ... Centuries ago a group of Elves committed an unholy ritual. But the ritual was a spectacular failure. It transformed the Elves into the Noctivagi - the beings we know as vampires. Striving to maintain a fragile peace, the ancient Elves and Dwarves negotiated an Accord, which they've renewed every year on the Summer Solstice. The ceremony depends on the magical Rune Stone, but it's missing. If it is not found—and quickly—the consequences could be dire, and not just for Elves and Dwarves. Four children, Rob, Jack, Eleanor, and Flora, stumble into a world of magic and mayhem when they move into Black Ledge, the old estate on the Maine coast, and discover they aren’t the only ones who live there.