Complex sentence: Dormammu briefly appears when his domain is entered by the newly appointed "Sorcerer Supreme" Doctor Voodoo, and he arrogantly dismisses the "unworthy" opponent and his vastly inferior "gutter gods".
Split into simpler sentences: Dormammu briefly appears when the newly appointed "Sorcerer Supreme" Doctor Voodoo enters his domain. The villain dismisses Voodoo as an "unworthy" opponent and refers to his vastly inferior "gutter gods".