The Battle for Cathora — Robert Cole

After many battles with a creature only known as Zelnoff, the children Mytar finally discover and transport the sixth Mytar from Earth to Cathora, completing the last member of their group. However, the Mytar powers are still underdeveloped and require intensive training to reach their true potential. In contrast, Zelnoff has gained more of the Mytar powers and his armies roam the planet unopposed. When a circling Zentor finds their hiding place, the Mytar children are forced to flee to the mistlands where they are ambushed and scattered.
With the situation seemingly hopeless, they discover critical information which points to a potential weakness in Zelnoff. With this information, one last desperate plan is devised. The Mytar must pool all their powers to fight an alien army and penetrate deep into Zelnoff's territory, if they have any chance of defeating him