Follow the cave and jump down into the water. Defeat the two Erynias using your crossbow. Go back towards where you jumped down to the glowing part of the cave and climb up the ledges. Follow the blood trail until you get to a ladder. Climb up all the way to the top, follow the cave and turn right at the intersection.
The troll’s riddle is:
“Lots eats, lots drinks. Big he be, steps big he, big he bash.”
The correct answer is number 3, a troll.
To find Morkvarg go to the back of the garden and into a hut:
After you speak with and fight Morkvarg pick up the werewolf meat in the pile near the entrance. Then go into the cave near the hut and fight Morkvarg again:
Afterwards feed him the Werewolf meat, which can be found in the quest items tab, to get rid of the werewolf in the garden.
To find the mask of Uroboros in Emiron’s workshop pick up the mug of mead:
Place it in the statue’s hand to open the door.