The Lost Prince — Bridget Wood

IN IRELAND'S DARKEST HOUR
The evil wizard Medoc ruled from the Bright Palace, spreading the corruption of the Dark Ireland across all the fair land. The time had come for High Queen Grainne--great-granddaughter of the Wolfking Cormac and Joanna, the legendary Time Traveller--to drive the usurper from the throne that was rightfully hers.
Grainne's armies made ready. Her generals plotted strategy. But against the wizard, their chance of victory seemed slim indeed.
Then Fergus, head of all the High Queen's forces, proposed an astonishing mission. He volunteered to travel into the perilous Far Future. There he would harness the fabled Apocalypse--the only power strong enough to defeat Medoc's Dark Ireland!
And while Fergus sought help in the Far Future, Grainne set off to find the ominous Castle of Shadow, the citadel from which travelers never returned. Something perilous was hidden there, something powerful--and Grainne was ready to brave any hazard to save her people from Medoc's malevolence...