The Tower Outta Nowheres - Skellige
Location: Urialla Harbor
Quest Giver: Automatic upon entering town
Other Requirements: N/A
This quest begins by arriving in the town of Urialla Harbor on an island in the northwestern portion of Skellige. In the center of town there will be a few townsfolk calling you over to speak with them - do so to be filled in on the back story for this quest. As the story goes, the tower magically showed up into town not too long ago and it has been non-stop raining since. Some people went inside to investigate and never returned.
Head on over to the tower (marked on your compass/map) and enter the portal. You'll be greeted by a Golem (pictured below) which you'll need to defeat before proceeding. There are a few things for us to do at this point, you can speak with Sigo Buntz (the man behind the barrier who calls out to you) and inspect the corpses on the ground using your Witcher Senses.
Golem Weaknesses: Dimeritium Bomb, Elementa Oil
Make sure you grab everything you come across in this tower. There is no one of a kind loot in here but there is quite a bit for you to find. Use your Witcher Senses if you're having trouble locating anything. Or use a Cat's Eye Potion to help yourself see in the dark.
Deeper into the tower when you get close to the "Find the key to the Library using your Witcher Senses" quest marker you'll fight a pack of Alghouls followed by a Gargoyle and a Werewolf. Once they're all dead loot the Key to Library in this room to update your quest. Use the key at the new quest marker on your compass (it's in the water) and in the next room you'll want to gather all of the loot you can then examine the books to find the grimoire you're looking for.
After discovering the book the room will be filled with gas, flee from this room and head back to the tower's entrance where you met Sigo Buntz to give him the book that you found. He will ask you to tell him when you're ready and then lead you deeper into the tower. Our next fight will be against an Earth Elemental and they're weak to the same exact thing Golems are from earlier.
Earth Elemental Weaknesses: Dimeritium Bomb, Elementa Oil
Throughout the fight there will be electricity sparking on the floor - it will damage you and the Earth Elemental if either one of you are standing in it. Focus on avoiding it but if you can use it to your advantage by pushing the Earth Elemental into it. Upon defeating the Earth Elemental there will be some dialogue with Sigo which will almost complete the quest.
You'll receive a Moonblade and then be teleported out of the dungeon, landing in the ocean. Swim to shore and speak with the villagers here to complete the quest and get your final EXP reward.