Blame the Bearer - Kingdoms Gone, #4

Vane has never turned away from an opportunity to seize power. He's spent his life manipulating others for his own gain. So when an ancient castle presents him with a magic ring and a legendary promise, he knows exactly what to do. Use the ring, find the Heir...and betray them at the first opportunity. Lidya is the middle child, neither lovely as her sister not physically able as her brother. She's lived in their shadows all her life and been happy enough there.
But when a man called Vane shows up with a magic ring and an insane tale, Lidya's life changes. Before anyone can stop it, she's trapped in a contest that will drag them all on a quest, put one of them on an ancient throne, and could easily cost all of them their lives.