Complex sentence: Despite these different imbodiemnts, the Wizard promises to grant their request, only on the condition that they prove themselves worthy of his assistance first, by successfully eliminating the Wicked Witch of the West who controls Oz's western quadrant called Winkie Country.
Split into simpler sentences: Despite these different imbodiemnts, the Wizard promises to grant their request, only on the condition that they prove themselves worthy of his assistance. He orders them to successfully eliminate the Wicked Witch of the West who controls Oz's western quadrant called Winkie Country.