Contract: The Mystery of the Byways Murders - Velen
Location: Oreton or Byways
Quest Giver: Oreton Notice Board or Exploring the Byways
Other Requirements: Wild Hunt DLC
You can start this quest one of two ways, the first and most common method is by accepting the contract from the Oreton Notice Board. Alternatively, you can also begin this quest by visiting the Byways town in southern Velen.
The first thing you will have to do for this quest is head over to the location on your map/compass called the Byways. You can also fast travel here if you've visited it before as there is a Signpost right here (warning: if you fast travel here you'll be thrown immediately into a fight with ghouls that are higher level than the other enemies in the area).
After defeating the ghouls in the village you will have a scene with two surviving villagers. Regardless of your choices while talking to them, examine the Nilfgaardian corpses on the ground to acquire a scent then follow it to advance the quest forward. This part is technically optional, but you should do it for the Trophy and Mutagen at the end of the quest. Alternatively, just go turn the quest in if you don't care.
The villager will try to stop you before going into the building the scent leads to, a conversation with him will get you the key to the locked door. Inside you'll find some fur on the ground and a trap door - Geralt will conclude that the enemy is an Ekimmara.
You'll find the Ekimmara, which goes by the name of Sarasti not very far inside of the tunnels. To save you the trouble with reading the Bestiary for this creature, it's weak to Devil's Puffball, Vampire Oil and Igni. Once you've defeated it you'll want to return to the villagers and speak to them for some experience then head over to the Nilfgaardian contract issuer for some more EXP and your reward.