Contract: Swamp Thing - Velen
Quest Giver: Downwarren Notice Board or Leslav
Other Requirements: N/A
There are a few different ways for you to accept this quest; visit the Downwarren Notice Board, discover the fog in the swamp or speak to Leslav (the person who issued the Contract). Venture over to the area marked on your map to be ambushed by a group of Drowners. Take them out then investigate the center of the quest marker by using Keira's Eye of Nehaleni which will reveal footprints under the rocks.
Note: You can do this quest without the Eye of Nehaleni but it's recommended you have it. You receive it by completing the story quest Wandering in the Dark with Keira.
Head on over to the new quest marker on your map and investigate the body and the blood (if needed) to update your quest again. Now depending on whether or not you have Keira's Eye of Nehaleni determines which entrance you'll be using to enter this cave. The quest assumes you have the Eye of Nehaleni but if you don't have it you'll need to use the entrance not blocked by an illusion. Use my screenshots below for additional guidance.
If you entered the cave from the illusionary entrance you'll fight Ignis Fatuus very shortly after entering. He's a named foglet who has his whole gut torn out. The fight with him resembles that of a noonwraith or nightwraith - he'll constantly vanish into the fog and summon some mist foglets to attack you.
Try to avoid as many of his attack as possible since he hits quite hard - even on the easiest difficulty. If you need a moment to regenerate your health leave the fog and that will greatly limit how much he attacks you since his magic (for the most part) only works in the fog.
Ignis Fatuus Vulnerabilities: Moon Dust (Bomb), Necrophage Oil, Quen