Start the quest by talking to Zaros in his restored throne room and he will bless you and your defence and attack will be doubled, then speak to Guthix and he will tell you to give him all four Godswords and once you give them to him he will combine all of them together with his power creating the Balance Godsword. Once you've defeated them head back to the throne room and you will see a cutscene of Zaros and Guthix fighting Zamorak. Zaros will stab Zamorak with the Staff of Armadyl and Guthix will blast him and give his power to Zaros.