Contract: The Mystery of the Byways Murders
Prerequisite: None | Suggested Level: 22
You can grab this contract from the Notice Board in Oreton. Do so, then speak to the Nilfgaardian commander in town, Milan Noran, to learn about a monster that's been killing near the Byways.
This area will be marked on your map, so head there to find a squad of ghouls. Take them out and a few townspeople will appear. They won't be any help, so use your Witcher Senses to inspect the corpses in the area. This will eventually trigger a bloody scent trail. Follow it to a nearby house, and you'll be stopped by a villager.
You'll also learn the truth about what happened here. Enter the house and use the trap door to enter a tunnel. Follow these to find the beast responsible for the murders: an ekimmara. Kill it - Devil's Puffballs, Vampire Oil, and Igni all help - and let the villagers know the threat has passed. Return to Milan Noran for your reward.
Contract: Swamp Thing
Prerequisite: Complete "Wandering in the Dark." | Suggested Level: 12
You can get this contract from the Downwarren Notice Board. Once you do, head to your marker and speak to Leslav, who warns of a monster prowling around the bog. The next step is to investigate the foggy area where it's said to lurk.
Use your Witcher Senses here to find some bodies; this will eventually lead to a misty red trail. Follow this to a blocked cave opening and use the Eye of Nehaleni to open the way. Head inside to meet the "swamp thing" itself: Ignis Fatuus. This battle can be tough - Ignis can deal some heavy damage - so stay on your toes and use effective tools like Necrophage Oil and Quen to come out on top. Once you do, return to Leslav to for your reward.
The Whispering Hillock
Prerequisite: Begin "Ladies of the Wood," needed to complete it. | Suggested Level: 5
This is listed as a side quest, although it really is part of the main story. In order to complete "Ladies of the Wood," you'll be sent to stop a dark force at Whispering Hillock. When you reach it, use your Witcher Senses to find a cave entrance. This will lead you to the source of the voice: a spirit bound to the plants in the area.
From here, you can either kill the spirit where it's bound or free it. If you choose the latter, you'll have to ride west to find the spirit's remains, then south to capture a wild horse using Axii. Bring these back to free the spirit, or kill it in a simple boss fight at your next opportunity. Whichever path you take, you can then speak to the ealdorman outside to start a long cinematic, wrapping up the quest.
Return to Crookback Bog
Prerequisite: Complete "Family Matters" and "Ladies of the Wood." | Suggested Level: 7
After completing these two story quests in Velen, the Bloody Baron will want to check on his wife. Head to Downwarren to join the Baron in his charge into the bog. Follow him and his men, taking out enemies until you reach the Orphans of Crookback Bog huts.
When you do, two scenarios could play out. You may encounter the Baron's wife in a crazed state or in a cursed, monstrous state. If it's the former, the quest will end after the next battle. If it's the latter, you have more work to do. In either case, you'll first have to battle some monsters as a dark fog rolls in; this includes a particularly tough Fiend.
If following the cursed quest path, you'll now have to deal with the Baron's wife in her monstrous state. Start by heading to your marker to find more baddies around Johnny's den. Take them out, then use your Witcher Senses to track Johnny's footprints to a small shelter where he's hiding. He'll explain what's happened to the Baron's wife.
Return to the Orphans of Crookback Bog and enter the Crones' hut. Look for a cellar entrance and climb down. You'll find some voodoo dolls the Crones have been using - take the one "with the violet hollylock bloom." After a rather sad cut scene, return to the Baron in Crow's Perch to complete his last, long quest.