A Confluence of Obsidian. The Obsidian Sisterhood, #3

The enemy of her enemy is not her friend ... but they may be her only hope. Sister Monica has defeated the dark Mages' attempts to seize the magickal stone she holds ... so far. But victory has come at great personal cost, and one of her own has been kidnapped. Now, in exchange for Talia's return, Monica's enemy demands not just the stone, but the ancient and powerful Methuselah to wield it for them.
If she hands them over, the Mages will have unimaginable, awful power. If she doesn't, Talia will die. But a second faction of Mages has emerged to offer Monica their help. She knows she can't trust them, but with no idea where to look for Talia, she may have no choice but to accept the alliance ... And then hope to hell she can beat them at their own power game before they destroy the very fabric of the universe.