Grail of Stars — Katherine Roberts

With evil cousin Mordred dead, Rhianna and her friends set out to find the final magical Light - the elusive Grail of Stars - so they can harness its power to help return her father's soul to its body. When the ghost of Sir Galahad guides Rhianna to the Grail Castle, she faces a series of dangerous tests and uncovers a shocking secret about Merlin. But when Mordred is sighted at the Lonely Tor and her best friend, Avalonian prince Elphin, goes missing, Rhianna finds herself fighting for more than just the Grail.

The final book in an epic series set in the Dark Ages.