An Echo in Time: Ancient Future Trilogy, cartea 2

"The narrow stairway inside the mountain led to a door that opened into a huge marble plateau. Upon this stood a stone circle of nine of the largest hunks of polished crystal Tory had ever seen. A turbulent cloud erupted overhead, its core alive with electromagnetic activity."

For twenty years the kingdoms of Prydyn, Dumnonia and Dalriada have prospered in peace under the guidance of their High King, Maelgwn of Gwynedd, and his good Queen, Tory. But when the High King is stricken with madness and a mysterious plague, Britain is set to be thrown into the chaos of yesteryear.

Tory seeks the advice of an advanced civilization that thrived long before recorded history, where men of great learning and miracles were abundant. The women of the Otherworld agree to aid Tory to make her passage back through time. But once in Atlantis, Tory must use her own devices to find the cure she seeks and a way to return to her loved ones in the Dark Ages.