Avallac'h - Through Time and Space
Avallac'h comes up with the idea to travel between worlds and speak with Ge'els, another general of the Wild Hunt. Once you agree to help him you'll need to travel to an area in Novigrad north of Hierarch Square, or accept Avallac'hs offer to fast travel here. You'll find a portal here that will take your to another world (pictured above). Follow Avallac'h once you're here to the next portal where sand crabs will attack you (pictured below).
Tip: You can endlessly farm the Sand Crabs here that attack you. They'll spawn infinitely until you enter the portal and proceed with Avallac'h
In the second world (a swamp world) you will be split up from Avallac'h and you'll have to find your own way over to the next portal. Shortly after going through the portal you'll encounter some toxic clouds on the ground in this world. You won't be able to use Aard or Igni on them so you'll need to instead climb up onto the rocks when you need to catch your breath.
Throughout the entire area with the poisonous clouds you'll find rocks and ledges you can climb up on to get out of the gas. Just after the gas you'll come across a Place of Power (pictured below) you'll want to make absolutely sure you draw from it as you can't return to this location once you've gone through the next portal.
After the Place of Power things will get much tougher as you'll have to sprint through the gas and you'll very likely take damage as you'll run out of breath. Have food prepared as well as Superior White Raffard's Decoction, they'll save your ass if you get too low in health. Keep heading south following the quest marker to reach the portal.
The next world you navigate will be entirely water; thankfully the portal you have to swim through is right in front of you. Once you go through that you'll find yourself in an ice world. At the end of the cave tunnel you'll come across an ice wall (pictured above) that you'll be able to Aard or Igni through.
Outside the wind will be blowing fiercly and you'll face a problem similar to the swamp world where it's so cold that Geralt takes damage just being out there. Take cover behind rocks or in crevaces as you head north towards the lighthouse that you see. You'll slide down a steep slope, go through a bunch of houses and eventually in one of the shed like buildings fight a few Hounds of the Wild Hunt. Try to stay inside the building when you fight them otherwise you'll take damage from the wind outside.
You'll be able to enter many of the building throughout the abandoned town on the way to the light house. Not only will you want to enter them to get out of the winds for a bit but you'll also find some pretty common loot in the crates/bags throughout as well.
Eventually you'll come out to a wide open area in front of the light house with three braziers (pictured above). Standing near a brazier means you'll not lose health from the cold (and have advanced regeneration from the brazier) - keep this in mind as you battle the few Hounds of the Wild Hunt that come out to attack you.
Once you defeat the hounds head into the light house to (basically) end the quest. All that's left of the quest is a long cutscene; after which you'll find yourself back in Novigrad