The Palidor and the Vathek are said to have coexisted peacefully for a long time, until a plague changed the Vathek into vampires. The Palidor were immune to the plague. The Palidor were overpowered by the Vathek and put into camps, where they were used as sustanance for the Vathek.
Together with a captured Dejah Thoris, a small group of Palidor rebels devised a way to dissolve the clouds above Vona, thus destroying the Vathek (as are most vampires, they are sensitive to sunlight). The cloud dissolving particles were ultimately launched by Dagur Andlaust, himself a Vathek, who was in love with Dejah Thoris.