The Warding of Witch World — Andre Norton

Estcarp faces apocalypse: in a battle of Powers the great key to the gates has been lost. The portals stand open; and pan-dimensional evil is poised to invade. ALL of Witch World's champions must band together and dare unknown dangers in a desperate race to find and close the planet's uncharted gates.

But. if they succeed, the Witch World will be shut off from the cosmos--forever.