Skilled in Magic - Five Go Into the Dark Worlds

Four orphaned children. A remote farm and a strange guardian.
Each child has a hidden gift and together they will become skilled in magic, a circle of sorcerers. But first they must harness their separate powers and uncover the mystery of their birthright.
The eccentric Professor holds secrets too. Through him, they learn the tragic truth of their parents' death and inherit a magic quest. Embarking on a reckless rescue mission to three dark worlds controlled by the evil sorcerer, Lord Mediarn, the children are tested by danger and heartbreak.
What is the greater magical power that they must claim? And why are five more powerful than four? Only by heroic measures may they hope to defeat the dark wizard and release the worlds he has held captive for so long.