Time Crystal 3 - The Icosahedron - The Eridon Chronicles, #4

Using her Crystal to gain passage through Eridon's force-field, the evil Warlord of Etiros invades the peaceful planet. W'taa Ofdrin Kitirrith's hardened warriors easily overrun the unprepared world and take the Great Council prisoner. However the man most wanted by the Warlord, Adelrid Merylon, inventor of the time-ships, manages to escape her clutches. Adelrid warns Heathcliff and his half-castes about Ofdrin's arrival, and they land in a distant forest and hide their ship.
Their plan is to get their Crystal to someone in power. However, on their their journey to the Eridon capital Saren, they are ambushed by Etiran warriors, and the Warlord forces Heathcliff and Charlotte to bring the stone to her. Gleefully Ofdrin unites their Crystal with her own, creating a truly enormous icon of power, strong enough to bend space-time itself. She is virtually invincible and unstoppable.
Aidan, Con, Kirsty and Katarine meet up with the Human/Eridon Half-caste Team and Adelrid Merylon, who has taken refuge with them. They believe the situation is hopeless until Con wonders if they can fight Ofdrin by joining psionically and using mindwave enhancers, outdated machines only kept for research purposes. Now the future of Eridon lies in the hands of four inexperienced half-castes, a team of mad scientists and one bad-tempered master mentalist.
If they fail, the Eridon people will be conquered and enslaved forever. Also available in the Eridon Chronicles:Half-casteTime Crystal 1 - The ConvergenceTime Crystal 2 - Delsaron's WorldTime Crystal 4 - The Singularity