Complex sentence: In the midst of the duel, Voldemort unsuccessfully tries to possess Harry in an attempt to get Dumbledore to kill the boy, then escapes just as Cornelius Fudge appears, finally faced with first-hand evidence that Voldemort has truly returned.
Split into simpler sentences: In the midst of the duel, Voldemort unsuccessfully tries to possess Harry in an attempt to get Dumbledore to kill the boy. Dumbledore does not do so and Voldemort escapes just as Cornelius Fudge appears, finally faced with first-hand evidence that Voldemort has truly returned.