The Way Between the Worlds - Ian Irvine - Bog

THERE IS A DARK FULL MOON on midwinter's day. The foretelling has come to pass. Rulke the Charon is unstoppable now.
Karan is held captive in desolate Carcharon tower. Rulke plans to use her to find the Way between the Worlds. On the mountainside below, the allies await their fate. Karan's lover, Llian, is in chains, falsely accused of betraying her to the enemy.
As the dark moon rises, Rulke begins to open the Way. If he succeeds, the world will be overwhelmed by the dread armies of the void. There is only one solution. Karan must be the sacrifice.