The Thirteen Hallows by Michael Scott, Colette Freedman

The Hallows. Ancient artifacts imbued with a primal and deadly power. But are they protectors of this world, or the keys to its destruction? A gruesome murder in London reveals a sinister plot to uncover a two-thousand-year-old secret. For decades, the Keepers guarded these Hallows, keping them safe, hidden, and apart from one another. But now the Keepers are being brutally murdered, their prizes stolen, the ancient objects bathed in their blood. Now, only a few remain. With her dying breath, one of the Keepers convinces Sarah Miller, a virtual stranger, to deliver her Hallow--a broken sword with devastating powers--to her nephew, Owen. The duo quickly become suspects in a series of murders as they are chased by both the police and the sadistic Dark Man and his nubile mistress. As Sarah and Owen search for the surviving Keepers, they unravel a deadly secret the Keepers were charged to protect. The mystery leads Sarah and Owen on a cat-and-mouse chase through England and Wales--and history itself--as they discover that the sword may be the only thing standing between the world... and a horror beyond imagining.