The Keepers' daughter - Gill Arbuthnott

Ten years ago, Alaric, leader of the Shadowmen, killed many of the
Keepers - the teachers and bearers of ancient knowledge threatening
his barbaric rule.
Now rebellion is flaring up in the Archipelago again and the Shadowmen
are out hunting for Keepers once more. Fourteen-year-old, Nyssa,
and her uncle, Marius, descendants of the Keepers, must flee,
but there is a greater reason that Nyssa is in danger: on her
skull is one half of a strange tattoo, three lines of writing in
a strange unintelligible script, the Legend of the Keepers.
Her search for the other half of the script begins, leading her down
a dangerous path from which she may never return.