The Doomspell Trilogy: The Wizard's Promise: Book 3 by Geoff Taylor
Replace the current stuff with this from Amber:-
The magic of all children has been released. Now, throughout the skies of Earth they fly, playing the deadly spell-games.
Only Rachel watches for what she knows is coming - the invasion of the Witches.
And this time there is a new enemy - the terrifying Gridda-breed. Rachel and Eric will need all their skill and courage in the coming battle - knowing that if they fail, the whole world will be engulfed in darkness.
The final instalment of the classic Doomspell Trilogy.