When you finish off Amdusias you will get access to Sentinel Island, where there is a bit of stuff to do before you get to the battle tower. Your first encounter on the island you can mindread to go up a level, so the higher level you are (I made it to 22 in the fjords) the more of an experience bonus that will be. You don't need to get to a high level, but it would be better if you got to a new level before entering, to get a full level bonus (equip anything you have with a Wisdom boost).
Eventually, near the end of acquiring the battle tower, you will need to finish off pretty much everything in Broken Valley and on Sentinel Island; there is a warning in-game before that point. You have to wait until after you have the battle tower before entering the quarry tower in Broken Valley and another obvious quest on Sentinel Island (where you will need to be able to fly to reach an objective); any other open quests in those two areas will fail.
You can talk to Lovis again before or after going to Sentinel Island, but only up until you take over the battle tower (the door will be sealed after that). You may want to save and try the quest both ways; returning the soul stone gives a better reward, but the dialog options if you destroy the stone are well worth the reload.
