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You can get this one by walking toward the side exit of the camp. When you walk past a square area on the lower level of the fort, you'll hear some men making fun of a soldier and refusing to allow him into their camp. If you talk to him, you'll find out that Mavrick had a bit of an accident during a run-in with a wraith on the shore.
While Geralt may not particularly care about preserving the man's integrity, a wandering wraith is something that should probably be looked into. If you want some backstory, you'll need to talk to one of the prostitutes that was around for the last battle. Just walk over to the brothel and you should find her outside the main tent, sitting by the fire.
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Investigating the House
With your new information and directions in hand for the quest, you'll naturally need to find the house that he's talking about. To get there, leave the main fort and walk through Roche's camp. If you cross the river and stick to the road, you should reach an area with a very noticeable house and a nice view of the beach.
If you look around, you'll notice a few things. In particular, there is a graveyard nearby with the names of his father and some younger siblings on the tombstones. You should probably make a note of this for a little side quest.
For now, I suggest that you just go ahead and walk over to the house. Use the Aard sign on the barrels near the house to reveal a trap door to the basement of the house. You can then go down and find a small puzzle. You're supposed to pull the levers in the order that's on the father's tombstone. The short answer is that it just goes left, right and up. There's no punishment for doing it wrong, you just have to fight a single wraith, which should be a piece of cake at this stage of the game. The reason for doing this is that you can get Malgat's notes, which is a little bit of backstory for the quest. It seems that he was involved in some fairly dark or strange stuff. There's actually nothing that we can do with these notes just yet. Dethmold is willing to buy them, but it's better to just hold onto them for now.
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Handling the Wraith's Request
With the side/hidden portion of the quest done, you should really confront the wraith. Just walk out to the beach and meditate until around 11 or midnight. The ghost should then spawn, and you'll be able to talk to her. She'll claim that Mavrick was actually the one who killed her, and that she's haunting him out of a thirst for revenge. You then have to make a bit of a decision.
If you want, you can just say that you think she's lying and fight her. She's actually a bruxa, and she'll spawn two “little sisters" who are basically just wraiths. Use Quen and then start swinging. If you manage to avoiding getting pinned, it's fairly easy to pick away at them with some specter oil and quick attacks.
That's actually a fairly disappointing end though, as you won't hear the truth and you won't get much extra experience for helping him.
If you want to check the facts, you can go back and talk to Mavrick. If you flat out accuse him of murder, he'll run off in anger and get himself killed. When you go to check the beach, the bruxa will reveal that it was all a trick and then summon a bullvore to fight you. Just keep moving and use some necrophage oil and you can probably be fine.
The “best" outcome seems to be asking Mavrick to come to the beach to answer some questions for you and settle the issue. If you walk there with him at around midnight, you'll get an option to pick sides here. Leaving Mavrick to the ghost will play out just as if you accused him before. Taking his side will cause her to summon the two wraiths (“little sisters"). The bright side is that you'll have Mavrick around as a distraction. With his help and some specter oil, you can easily take them down. Once they're dead, Mavrick will thank you and tell you the whole truth about the deaths of his family.
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All information and screenshots come from Witcher 2.