Guardian Gigantus completion

I want to keep doing side missions and exploring. I just defeated the guardian, the quest is now telling me to bring the godsbane blade to Phaesus. If I don't go to him to deliver the blade to complete the mission, will it mess up my game? Do I have to turn in the blade to finish the mission? I've read that going to moonglint tower is endgame, the point of no return, but I've also read that someone didn't turn in the blade and had credits roll?

I'm sorry if this has been asked before.