Complex sentence: Jeroen was cured of his heroin addiction when Midnighter gave him a transfusion of his blood and later relinquished his powers to the renegade Doctor in order to save the Earth, regaining them when the renegade Doctor was killed.
Split into simpler sentences: Jeroen was cured of his heroin addiction when Midnighter gave him a transfusion of his blood. He later relinquished his powers to the renegade Doctor in order to save the Earth and regained them when the renegade Doctor was killed.