Right after Blood Run ends you will very likely run into a boy who will deliver a letter for Geralt, from Yennefer or from Triss but only if you romanced her, otherwise it's always from Yennefer, but it's different depending on whether you romanced her or not. Now you will have the option to tip the boy with 5 or not.
Important: If you tip the boy it will count as "Proof of generosity" for There Can Be Only One (so just pay it you cheapskate)
Go to the Cemetary at Lebioda's Gate the grave you are looking for is just west from the fast travel point. However, someone has robbed the grave so investigate with your witcher senses to find footprints and follow them, but the trail ends real quick with no way to continue the search. Now open your inventory and use the Old map of Toussaint which will tell you where to go next. Go to the new location and dive underwater, you will find a portal there so enter it and you will appear in the ruins. Soon you will find spike traps, but you don't need to cross them unless you want some loot, instead start climbing down, along the way some panther statues will come to life and attack, kill them and keep going. Along the way you will run into evidence that the grave robber is there too. Eventually you will reach a closed door and to open it you will have to shoot blocks on walls with your crossbow to open it. Inside you will find a pedestal and Gargoyle statues, after you examine one it will come to life and you will have to fight it. After you beat it it will drop a hand which you have to place on a pedestal. Portals will open, take the left one, now climb down and climb up to the portal the statue is facing, enter it then keep moving left until you see a gap, jump over it and enter the portal you find there, this will take you to the lab.
Search the lab and you will find a Megascope, however don't use it yet instead search the place to find 2 crystals, one is on the desk, the other on a bench behind the machine that looks like an iron maiden. Go back to the Megascope and use the pillow nearby that has crystals on it and place the remaining 2, this will give you the optional objective of checking the 2 additional recordings. Now activate the Megascope and listen to what Moreau has been up to. As it tuns out he tried to cure the mutations of a witcher to turn his son back to normal, however instead he strengthened them. To perform the process giant centipede eggs are required, thus Geralt decides to search the place for them. Giant centipedes are tough but you can entangle them with Yrden making the defenceless. After you reach the nest take the Mutated giant centipede albumen, only 1 dose is needed, now take the other corridor and you will reach a breakable wall, use Aard to break it and you will end up back in the lab. Now place the albumen in machine that looks like an iron maiden, now strip and activate it. With this Mutations have been unlocked. Now go to the new marker and activate the crystal with Aard to open the portal, enter and it will teleport you to the lake where the entrance portal was and the quest ends.
- Default (didn't romance or doing standalone):
- It was a warm summer evening... or perhaps a cool yet sunny morning? The weather and time are of no import, what matters is that day Geralt received a letter from Yennefer, who informed him about a most unusual discovery she had recently made. She'd found mention of research conducted by a Professor Moreau on witcher mutations. It seemed this professor's workshop could be found in Toussaint. Geralt, realizing the importance of this discovery, decided to find this laboratory. The first step would be to find the professor's journal which, according to Yen, had been buried with him.
- If Geralt romanced Yennefer in the main game:
- It was a warm summer evening... or perhaps a cool yet sunny morning? The weather and time are of no import, what matters is that day Geralt received a letter full of kind words from Yennefer, who informed him about a most unusually[sic] discovery she had recently made. She'd found mention of research conducted by a Professor Moreau on witcher mutations. It seemed this professor's workshop could be found in Toussaint. Geralt, realizing the importance of this discovery, decided to find this laboratory. The first step would be to find the professor's journal which, according to Yen, had been buried with him.
- If Geralt romanced Triss in the main game:
- It was a warm summer evening... or perhaps a cool yet sunny morning? The weather and time are of no import, what matters is that day Geralt received a letter full of kind words from his beloved Triss, who informed him about a most unusual discovery she had recently made. In some old manuscripts she'd found mention of research conducted by a Professor Moreau on witcher mutations. It seemed this professor's workshop could be found in Toussaint. Geralt, realizing the importance of this discovery, decided to find this laboratory. The first step would be to find the professor's journal which, according to Triss, had been buried with him.
- Default (didn't romance or doing standalone):
- Geralt soon discovered not only he wished to find Professor Moreau's laboratory. When the witcher reached the cemetery, he found the professor's grave had been dug up and the journal was nowhere to be found. The only clue he did find was an inscription on a grave mentioning the legendary Dol Naev'de, the Valley of the nine, which had been flooded years before by the Sansretour River. Seeing no other way to find the professor's lab, Geralt decided to search for more information in those sunken ruins.
- As unlikely as it sounds, Geralt found the entrance to Professor Moreau's laboratory in the sunken elven ruins of Dol Naev'de. Things seemed to be getting more and more interesting, so the witcher did not hesitate to plunge onward...
His path was riddled with traps and other such obstacles, but our brave hero had much experience in running even more difficult gauntlets. Tired in both mind and spirit from his efforts, in the end he made it to the professor's lab.
- Searching Professor Moreau's lab gave Geralt a somewhat expanded view of the professor's past and his research. It seemed Moreau's goal had not been to discover how to strengthen the witcher mutations, but just the opposite, how to reverse them, so he could "cure" his son Jerome of being a witcher.
- To Geralt's delight, the professor's plan failed to achieve its desired result. This failure could be of use to the witcher, however, for by mixing the professor's secret formula into his bloodstream he gained access to a whole range of new mutations. These promised to make him stronger, faster, more resilient – in short, better. Geralt looked forward to putting them to the test in combat.
- Search for Moreau's grave at Orlémurs Cemetery using your Witcher Senses.
- Use your Witcher Senses to look for clues about the location of Professor Moreau's laboratory.
- Use your Witcher Senses to follow the grave robber's trail.
- Check the location of Naev'de on the map Yennefer / Triss sent you.
- Search for the entrance to the professor's laboratory in the flooded ruins of the Valley of the Nine.
- Search the ruins.
- Find a way into Professor Moreau's laboratory.
- Go deeper into the ruins.
- Find a way to open the doors.
- Find Professor Moreau's laboratory.
- Find a path through the portals.
- Use your Witcher Senses to search the laboratory.
- [Optional] Find the missing megascope crystals. 0/2
- [Optional] Place the missing crystals on the pillow. 0/2
- [Optional] Listen to the additional entries in the professor's audio log. 0/2
- Acquire mutated giant centipede albumen.
- Get undressed and activate Moreau's contraption.
- Leave Professor Moreau's laboratory.
- The name of this quest is reference to quote from the song by David Bowie named Changes.
- In the area with the portals is a portal that's blue but also glows orange and, upon entering it, the room has a crate painted like the companion cube and a nearby container with cake, all clear references to the video game series Portal.
- The quest has references to The Island of Doctor Moreau, a novel by H.G. Wells. In the novel, Doctor Moreau is attempting to genetically alter animals into more human-like beings, through a process called vivisection.
- The mutation contraption and some of the journal entries might be references to Captain America: The First Avenger.
- The quest makes several references to the Tomb Raider series.
- The achievement you unlock for completing this quest is called Weapon "W". It is a reference to X-Men character "Wolverine" a.k.a. Weapon X.
- When Geralt talks about the statue and its orientation, he's referring to the giant statue of a hooded figure (not the gargoyle sculptures in the first room).
- Enter the portal the giant statue faces to clear the puzzle.