Rias spell book quest NPC

[Decrapio]

I have the spell book, but no idea where the NPC to cash the quest is. Does anybody know?

Also, I have no remaining mainline quests, and have no idea where to go next. Have cleared everything I can find on the mainland and starting island. Any idea where I'm supposed to go next?

Devs, NPC tracking after quest completion, or at least listed area on where to go next would be greatly appreciated. At least 20% of my game play is wandering around wondering where I go next or where to cash a quest.

[Pirate]

If in doubt, check the resistance hideout in the sewers.

Once you get there, most of the main story quests will be from an NPC there.

And yes, Ria is there.