Tether - The Gateway Trilogy, #3

The end is near... Ember was only trying to keep a promise when she jumped into the demon world. But instead of saving Cole and his people, she finds herself just as trapped as they are. She lives and learns with the Daemon survivors while the demon threat grows every day. In the human world, Taren is tormented by guilt and facing new threats. He's desperate to know Ember is alive, but there's no sign of her except in his own vivid dreams.
As they struggle to reconnect, the Gateways around the world weaken, and the demons amass for war. The end is near and Ember must face her greatest fear if she has any hope of saving the world - or herself. With everything falling apart, her only hope is to find...her Tether.