The Fall of House Reardon - Velen
Quest Giver: Lindenvale Notice Board
Other Requirements: N/A
The first thing you need to do for this quest is to speak with Dolores Reardon in the town of Lindenvale; she's close to the Notice Board. Speaking to her will update your quest (and mark the location of the manor on your map) as well as get you a key which you'll be using at the end of the quest for your reward.
If you've linked a save file from Witcher 2 to this game then depending on what action you chose in that game towards Letho determines whether or not there are Wraiths at this manor or Letho. For those of you who didn't link a save file to Witcher 3, you'll always have Wraiths at the manor.
Technically all you have to do for this quest is defeat the 5 or so Wraiths at this manor to complete it - however there is an optional bit which you can do that will net you more experience and loot. Inside of the northwestern most building of the town you will find a chest (pictured below) which contains a Sturdy Barn Key. If you suck with directions, it's the second largest building in the town.
Once you have the Sturdy Barn Key you'll want to go behind the barn, which is the southern most building in town and also the largest building. Use the Sturdy Barn Key to gain access to the tnside of the "house" portion of the barn. You'll find the lockbox that Dolores was telling you about in here; it's found right under the writing on the wall that glows red with your Witcher Senses On.
There is a stairwell right behind the barn that goes underneathe of it which you're going to want to take afterwards. Loot everything in the basement using your Witcher Senses and blow open the weak wall (it's glowing red) with Aard. You'll uncover some scraps of paper in here which illuminate the rest of what happened here.
Before leaving the manor head out the northern end of the area and you'll stumble across a green fast travel Signpost. Use this to zip on back to Lindenvale and turn your quest in. Dolores can be found inside of the tavern of this town now.