You only get 12 ability slots over the course of progression to equip skills in the base game. Once you hit level 30 in The Witcher 3: Wild Hunt, you are likely going to be searching for ways to get more slots. Unfortunately, you cannot unlock more slots, at least not in the base game.
The Blood and Wine expansion pack introduced a new character upgrade system called Mutations. It allows you to create new bonuses and passives, and most importantly, unlocks additional skill slots.
That is the only way to get more slots in The Witcher 3. However, Mutations are initially locked. You need to complete a certain quest and achieve certain objectives before you get access to Mutations.
The following guide will tell you how to unlock Mutations and how to get more ability slots in The Witcher 3: Wild Hunt.
Turn and Face the Strange walkthrough
Turn and Face the Strange is a secondary quest that will begin automatically after you complete Blood Run. You will receive a letter from either Triss or Yennefer, depending on whom you romanced in the base game, to get the ball rolling.
Find Moreau’s grave
The letter will mention research conducted by the late Professor Moreau on witcher mutations. There will also be an Old map of Toussaint as a quest item.
Once you have read the letter, make your way to Orlemur’s Cemetery south of Temple Cemetery to find his grave. Unfortunately, someone has beaten you to it. The grave is empty.
Use your Witcher Senses to find and follow a trail of footprints that will lead you nowhere. So, open your inventory and check the Old map of Toussaint for more clues.
You will get a new location to go to. Head over there and dive into the water to discover a portal that leads you to the ruins.
Enter the right portal
There are several traps and dangers here for you to avoid. You can cross the spike traps for loot before climbing down to find Moreau’s diary as another quest item with more clues.
Make your way to the end where there are Gargoyle statues. One of them will come to life. Kill it and the statue will drop its hand. Place the hand on the pedestal to open several portals. You now need to enter the correct portal. Simply see which portal the statue in the center is looking at.
Search the laboratory
You will now find yourself in a laboratory with all sorts of equipment. You need to find a Megascope and two Megascope Crystals using your Witcher Senses. The first crystal is on the desk. The second is on a bench behind the machine.
Return and activate the Megascope to learn that Professor Moreau was trying to find a cure for the witcher mutation. However, his research only buffed the mutation further.
Perform the research to unlock Mutations
You now need to repeat his research in the laboratory. To do so, you need some Giant Centipede Eggs from a nearby cave. Use the hole in one of the cages to reach a Giant Centipede nest. Interact with the eggs and you will get Mutated giant centipede albumen as a quest item.
Get back to the laboratory and place the quest item in the mutation machine. Activate the machine and you will unlock Mutations in The Witcher 3: Wild Hunt. You will now have four new ability slots.