The Fatal Gate Av Ian Irvine

The cruel Merdrun, the greatest warriors in the void, have invaded Santhenar but their portal failed, leaving them stranded. They're desperately trying to regain contact with the summon stone so they can reopen the portal and begin the slaughter of humanity. Llian, hurled through another portal, sees the summon stone wake and knows the allies have but one chance to destroy the Merdrun - though it will involve an alchemical quest that has always ended in ruin, a mad invasion in untested sky ships with untrained pilots and the most unequal battle in all the Histories.