HADES is coming for the spire, and it’s up to you and your friends to defend it. Head for the Palace of the Sun then talk to Sun-King Avad. When the conversation is over, talk to Blameless Marad. Once the conversation is over, you can either go talk to Erend at the Spire and check on the defenses to the West, or just go straight to rest before the battle. We’d advise going to check the defenses at both locations as it will give you some extra story, as well as getting you familiar with the Oseram cannon you’ll be using to defend during the attack.
When you’re ready, go rest at Olin’s place. Note, this is the point of no return with the game, so if you want to do more before the final battle, do it now. Having said that, the game does put you back to just before the final quest, with all your exp earned during it, once you’ve finished the game to continue playing. So it’s up to you really. Whatever the case, rest and that’s this quest done.