The Witcher 3: Blood and Wine unlocks the Mutations System when you complete "Turn and Face the Strange", a unique side quest in Toussaint with a reward you need to get ASAP. The Mutations System unlocks 4 new ability slots, while also allowing you to use a Mutation that has vast power, depending on your preference for weapon, defense, and sign. Below, we'll provide a full walkthrough of "Turn and Face the Strange" along with its many puzzles, then explain the Mutations System.
How to Start "Turn and Face the Strange" (& Should You Tip the Boy?)
You can start the Turn and Face the Strange side quest after completing "Blood Run", the third main quest in the Blood and Wine expansion. Just make sure you're wandering around Beauclair in Toussaint, and a random messenger should approach and give you a letter (from Yennefer, or Triss if you romanced her).
Once you receive the letter, you're met with two options:
Stamp says "postage paid," so get lost. Geralt refuses to give the boy a tip, and the boy leaves.
(5) Here, your tip. Geralt tips the boy, who is very grateful. This counts for the "Proof of Generosity" task in "There Can Be Only One".
You should definitely tip the boy, as it's only 5 Crowns (and at this point you should have over 50,000 Crowns). This is also the easiest way to complete the Proof of Generosity task in a different side quest.
Yennefer (or Triss) will ask you to visit Orlemurs Cemetery near Lebioda's Gate (as marked on the map above), but when you enter the grounds, you'll find that the grave has been robbed.
You will need to use Witcher Senses and follow the footprints, but the trail goes cold. Open your inventory and use the Old Map of Toussaint quest item, which reveals your next location, where you can dive underwater and access a portal within the ruins.
How to Find Professor Moreau's Laboratory
Inside, you will want to follow the objectives through the tower, avoiding the floor spike traps (Quen mitigates the damage), and you will soon fight Guardians, which are spectral panthers weak to silver swords. Continue through the ruins below, where you will find a dead-end room.
Locked Stone Door Puzzle
Inside this room, there will be four stones on the walls that you can shoot with your crossbow. There's no time limit, so look at the stone squares in the images above and shoot them. They are all in the room with the locked door, so check the walls!
Gargoyle Puzzle
This will open the stone door and allow you to enter a circular room. Examine any of the Gargoyles, and one will wake up. You must kill it and loot the Gargoyle Paw, which can be placed on a pedestal by the large statue in the center of the room.
Portal Puzzle
Three portals will open, and you can find the correct portal by entering the one where the statue is facing:
The first portal is the one when facing east.
The second portal is the north one on the second floor, which you can only reach by dropping to the lower floor, then by climbing the stones.
The third portal is to the left on the top floor, so keep going left and jump across the large gap where you can enter the west portal.
Search the Laboratory
Once you're inside the lab, you will want to search for Megascope Crystals:
One is on the desk with a book.
One is to the right on a bench by a large glass tube.
In the south area of the lab, there's a pillow that you can interact with to place these Megascope Crystals down. With that done, interact with the megascope to click "use" and then select the five entries to watch the available recordings.
You can now enter the cell to find a cave that has three Pale Widows (giant centipedes), and with them killed, you can grab the eggs at the back of the cave to collect the three Mutated Giant Centipede Albumen items. Now, return to the lab and place the item in the iron maiden device. Finally, take off your clothes and weapons (including the crossbow) and enter the iron maiden.
How to Use Mutations in The Witcher 3
This process will unlock the Mutations System, which allows you to have four more skill slots, provided you unlock Mutations. To begin, open the Character menu and press Y/Triangle to access the Mutations tab. Inside, you can spend Ability Points and Greater Green/Blue/Red Mutagens to unlock Mutations, but only one can be used at a time. However, purchasing multiple Mutations unlocks new skill slots to use:
Slot #1 requires 1 Mutation.
Slot #2 requires 4 Mutations.
Slot #3 requires 8 Mutations.
Slot #4 requires 12 Mutations.
This means that you can have a total of 16 abilities active at once!
You can now leave the lab by using Aard on the portal's crystal, which will teleport you back underwater, so rise to the surface and leave the area to complete the quest and start your next adventure!
Source: Game Rant