This Level 15 sidequest in Skellige can get triggered as part of the Nameless main quest.

Alternatively, you have to talk to the priestesses of Melitele to learn what happened in the garden. Then talk to priestess Josta about the monster, Morkvarg. Follow your map to find Freya's Garden. If you're coming back later, it's in the norse of the eastern-most island in Skellige. If you didn't do it as part of Nameless, you now have to find Morkvarg in the garden.

To find Morkvarg, follow the wolf tracks to cross a small river. They start inside the garden at the locked gate, at the southern-most point. You will hop over a few walls in your pursuit of the prints and over a small bridge over a canal. Nearby, there is a stone ladder getting you up to an area with switches.

Floodgate Switches

Use your witcher senses to find two levers that open sluice gates. There are three gates. The right lever controls the pegs in the gate construction that lets you pick the gate. The left peg raised means the left gate will open, and so on. The left switch raises the corresponding gate. This will allow you to access three areas. If you get here as part of Nameless and do it for the first time, simply pull the left lever and jump in the water and swim through the open sluice gate in the middle. To get back up after you've fallen into the water, look for a stone ladder that lets you climb out. These are a little hard to spot.

Facing forward in between the switches:

1. The left-most floodgate lets you exit from the garden.
2. The middle floodgate lets you get to Morgvark
3. The right floodgate gets you to a crucial quest item

Meet Morkvarg (Middle Floodgate)

If you swam through the middle gate, climb out at the end and mark your way to a shack with a torn-off door. If you get lost, this shack is at the northern end of the map. Use your witcher senses to examine the area and inside. You will now meet Morkvarg, who will tell you about the curse. Fight him (if you're low-level, remember your oils and potions) and he'll tell you that he'll come back again and again and everything he tries to eat will turn to ash. He also tells you that biting off his own limp caused his throat to burn.

This is a hint: you can pick up his flesh -- and feeding Morkvarg his own flesh will actually kill him. If you don't want to bother with the quest, this will end it fast and net you 85 coins (the same reward Josta would give you). But let's assume you want to actually complete it -- and there is a second reward in it for you -- so finish him off and loot his body. Note that you can repeat this kill to get more werewolf mutagens and meat. Just meditate until the wolf reappears. 

Talk to Einar

Fast-travel or run back from Freya's Garden to Larvik to talk to Einar about the curse, as suggested by your quest notes. Einar hangs out in the harbor. You'll find out that archpriestess Ulve cursed Markvarg over a terrible crime. But now that you know that, it's a waste of time, so don't bother. 

Go back to the switches and follow your marker to a cave just outside the garden to the north. You'll bump into Morkvarg again and overhear him talking to himself about "Rramund" and a key that he took. Examine the cave with your witcher senses to find about his friend's fate and learn about a hidden cave. There's also a book about polymorphy here. 

the Key (Right Floodgate)

Head back to the floodgate levers in the north-east part of the garden. Pull the right switch and then the left one to open the right floodgate. Jump in the moat and swim to the waterfall at the open gate. Use Cat to see in the dark and dive through a tunnel. At the end, you will find a cave with a treasure box and some minor goods as well as a pile of bones and a key. Swim back and exit the moat via the stone ladder on the left. 

Unlock the Chapter House Doors: Morkvarg's Journal

Back to the switches. Raise the middle peg and swim through the open gate to get back to the Chapter House near the big tree. There's a building to the west, about midway at the left/western wall of the garden at the edge of the circle that's marked on your map. Open that door, turn left and climb up. (You can't open the door with the scratches.) Continue on and you'll get to the Chapter House. Inside, there are two doors. Open the right door to get some loot, including a Longclaw diagram.

Open the left door to get more loot and Morkvarg's Journal. You'll find out that Morkvarg was not an awesome person and has not been telling you the truth.

Back to Einar 

Fast-travel back to Larvik and confront Einar via the "Lied to me" option. Einar was one of Morkvarg's men. Tell Einar that you don't have to fight and tell him that you want to lift the curse (that's what witchers do). Ask him to do it for the priestesses and you get the necklace from him. Back to your wolf friend. 

Get Rid of the Werewolf in the Garden

Follow your marker to find Morkvarg in the garden. Remember, you've got the key, so you can open the door on the left (not the main gate) to get there faster. If the marker is off on the outside, that means Morkvarg is in the cave. Open the gate near the cave entrance (north-east) and find him. Fight the werewolf, say your "here we are agains," and pick the option to give him the Toradar's fang necklace.

You'll find that Morkvarg is unrepentant. Ask him for your reward to get him to tell you that you can get some coin from a pawnshop owner in Novigrad. Quest over, he walks away... You know what to do. Draw your sword or punch to start a fight. After you're done, loot his body for a sword. But mostly just to feel a little better about wasting your time trying to save this jackass. Note that this is optional. Josta will pay you no matter what.

Head back to Josta and ask for your reward: 85 crowns.

For your second reward, head to Novigrad and talk to the Loan Shark in The Bits. After selecting "Morkvard Sends His Regards", you will get experience points, Deithwen, and a torn-out page to brew the Werewolf Decoction.