Find this Level 30 side quest in northeast Skellige in Urialla Harbor on An Skellig. Hang out in the town square and a Fisherman will offer up the quest.

He tells you about the tower to the west, just under 200 steps away. Head towards it and enter. You will find a portal.

Enter the portal and you will be greeted by a not-so-friendly voice, telling you to get out. Use Cat to see in the dark and get ready with some Elementia Oil and Dimertium Bombs -- you'll face a level 29 Golem.

After you kill the Golem, look for a man trapped behind a magic barrier. From the magician Sigo Buntz, you will learn that you have to disable the Defensive Regulatory Magicon -- basically, to turn off the tower's security systems. Go look for Gottfried's Treatise. Go to the lab to get the key first.

Go to the portal on the opposite side and the path will open. The tower is now trying to kill you with flooded passages and level 21 Alghouls, a Gargoyle, and a Werewolf. Use Cursed Oil and Igni to take out the latter (and get a rare Werewolf Mutagen). When you're done, beware! A portal opens and... Nope, just a cow.

Look for a small locked chest near the alchemy setup. It contains the key to the library.

Head back to the flooded passage and open the door underwater. Use your Witcher senses to find books -- one of them, around the corner, is the Gottfried's Omni-Opening Grimoire you're looking for. Read it and quickly explore and loot the rest of the library. The defensive system is trying to gas you. When your breath is low, just exit the library and you'll be fine.

Back to Sigo. Give him the book and he'll open the barrier.

You now have to make your way to the heart of the tower. Sigo will deactivate the barrier and you'll enter a room that'll soon be home to a Level 27 Earth Elemental. Use Elementa Oil, bombs, and whatever you have to bring it down.

Once the battle is done, be careful! The moment you talk to Sigo, the quest ends and you'll be unceremoniously "exited." Loot the room for some runestones and weapons first. Then talk to Sigo and receive your exit door and a Moonblade. Talk to the townsfolk to finish the quest. Choose a reward (65 crowns) or take just their thanks.