Durst House

The Durst House is just outside the East Gate of the Valley, and was once an inn welcoming visitors to the valley, in the days of Harmony. The Dursts moved in shortly after Strahd took over, as they had formerly lived in his county. They converted the inn to a mansion, but still tasked themselves with extorting those who came to Barovia. Shortly after moving into the Valley, however, things began to go downhill.
Gustav began having an affair with the nursemaid Lada, but kept this secret from his wife. After about six months, however, Elisabeth found out and her already shaky mental health took a hit. She began working with the "Dark Powers" (Canis), hoping both to restore the beauty she thought she'd lost and exact revenge, and was tasked to give blood. She sacrificed every servant in the house (except the nursemaid, who her husband doted on and protected), as well as several guests of the house entering Barovia. She kept it a secret from her husband, and locked her children in their room while she did her business, and told them not to tell their daddy about the "monster" in the basement.
When Lada gave birth to a baby, Walter, everything came to a head. Mrs. Durst forbade Lada from removing the baby from his room, and began forcing her husband to study religious texts preaching faithfulness and harmony in marriage.
As Walter's first birthday drew near, Mrs. Durst's mental stability began to severely deteriorate. One night, she killed Lada in a fit of rage, and took the Walter to the basement. She sacrificed him to Canis, an act that twisted her and the guests she had waiting to be sacrificed into a ghast and four ghouls respectively. When Mr. Durst discovered what had happened while searching for his wife, he killed himself. Rose and Thorn starved to death in their room, having been locked in there hours earlier by their mother.
Since then, the house has maintained its original purpose: welcoming newcomers to Barovia. It uses the specters of Rose and Thorn, given the knowledge that they have an infant brother, to lure folks in.
The house is identical to the book except in the following areas:

9. Secret Room

There are no cult books. There are instead many blank books, and one book in which Elisabeth Durst had begun to write a religious text for Canis. She did not specifically name Canis, merely referring to them as "the Powers." She also kept a note of the servants and guests she sacrificed to Canis, and the gifts and visions she received in return.
The text to Strahd's letter is as follows:

My most pathetic servant, I am not a messiah sent to you by the Dark Powers of this land, nor am I a shining light in your misery delivered by Reve. I will not lead you on a path to immortality, salvation, nor reprieve from your misery. However many souls you sacrifice in my name, however many visitors you waylay from my roads, known that you are not what has brought me to this beautiful land, nor are you what keeps me here. You are but worms writhing in my dirt. You say that you are cursed, your fortune and youth spent. Your love has forsaken you, sired a bastard under your own roof. Lift you up in darkness? I think not. I much prefer you as you are. Your lord and master, Strahd von Zarovich

12. Master Suite

The double doors to this room have dusty panes of stained glass set into them. Designs in the glass resemble windmills. The room is pitch-dark when the characters enter, and as their eyes adjust, they see a gruesome sight: a bloated, blue body hanging from the ceiling, a discarded plush chair at his feet. For a moment, his body swings as if he has just stepped down.
Folded on the bed is Gustav Durst's suicide note:

My dearest children, Please forgive me. I have failed you. I wish I could do what all fathers do and tell you monsters aren't real. But I can't lie to you more than I have. But I will tell you one truth: monsters are made. Hardship, heartbreak, disappointment...these can all make monsters. And I have made a monster of your mother. All I can do is ask you to forgive me, one day. I don't deserve it, but I beg of you to think kindly of me one day. I made my mistakes, but I truly loved you. Rose, I wish I could see you blossom into a strong, beautiful woman. Thorn, I would have seen you be a great man, greater than I, kinder and stronger. But I won't. I am sorry. This is the only way.

14. Storage Room Guest Room

Rather than storage room, this is a bedroom for visitors, matching room 17, minus the doll.

15. Nursemaid's Suite

The ghost of the nursemaid, whose name is Lada, is not aggressive or hostile. She is a quiet, solemn woman who likes to make others smile. She is a confused ghost, torn between the duty of the house to attract guests and her own violent death. She shifts between knowing she's dead and not, but she never remembers how exactly she died. She speaks fondly of Mr. Durst, remembering him as a mild, kind-hearted man with a strong sense of duty. If the players try to inform her of his body upstairs, she will refuse to even acknowledge it. She will not under any circumstances admit that Walter is Mr. Durst's child, but she will claim him as her own son. She also speaks lovingly of Rose and Thorn.
If asked about Mrs. Durst, however, she becomes quiet and less talkative. It is obvious she is afraid of Mrs. Durst, and has a secret she doesn't want her mistress to know.
She doesn't become aggressive even if the players disturb the crib, but she will insist they let the baby sleep. The shroud on the crib is pale green, not black. If the players pull it back, the bundle-and Lada- vanish.

18. Storage Room

The specter never appears, even if the players disturb Lada's corpse. Lada's ghost can be encountered in Room 15.

20. Children's Room

On Rose's nightstand is a toy very similar to the Mockingcat: it is a raven with the paws and tail of a cat. When the handle on its back is wound, it opens its beak and plays a gentle, soothing tune. It has no magical effects. On its foot is a well-preserved tag that reads: "A creation of Friedrich Von Weerg, mastercrafter"

23. Family Crypts

There is no crypt for Walter. The crypt marked for him in the book is blank.

24. Cult Initiates Quarters

This room has been removed.

25. Well & Storage

This room has been removed.

27/28. Dining Hall/Larder

This is not a dining hall, it is just another storeroom, this one dedicated to food. There is no grick.

31. Dark Shrine

The statue is not of Strahd, but of Canis. Describe it as such:
"A wide alcove in the south wall contains a painted wooden statue in the crude shape of a massive dog with four heads stacked grotesquely on its wide shoulders. Each head is a different shape, and their mouths hang open. The lowest-hanging head holds in its mouth a smoky-gray crystal orb."
If a character touches the statue, or take the orb from the dog's mouth, a monster spawns, but it's not a shadow, but a Bestial Huntsman.

33. Den

The mimic has been removed from the door. This is essentially a date room.

34. Basement Quarters

This room looks more like a neglected sex dungeon than a bedroom.
There is only one ghast, and it is the ghast of Mrs. Durst. She is wearing a tattered red dress with a black smock. She is a character, not a mindless enemy. She’s retained her memory but has also completely succumbed to her dark whims. She’s arrogant and believes herself better than everyone, looking down on the players. She shuns her dead husband and calls him a lecherous traitor who deserved to die. She has even less kind words for Walter and the nursemaid. She even writes off her own children as bothersome nuisances.

35. Reliquary

The chant is in a language none of the characters can understand.
The items here are not appropriated by the "cult," but were kept by the Durst family as a Strahd's vassals.
36. Prison
This room has been removed.

38. Ritual Chambers

The place where the shambling mound would normally be is full of garbage. The apparitions that appear are black-robed figures with dog heads. They speak in a language no character can understand.
Whether the players make a sacrifice or not, the final enemy will appear. If they do sacrifice, the figures begin to bark delighted laughter, and a monstrous being rises from the water. If they don't, the figures begin to growl louder until it becomes a dull roar, and a monstrous being rises from the water. This is Walter, hideously twisted by the grotesque fate he suffered.

Post Boss Fight

When the players defeat Walter, the house is not trapped.

Statblocks

Flesh Mound (Walter) CR: 5

Large undead, any

Armor Class: 15

Hit Points: 136 (16d10 + 48)

Speed: 20 ft , swim: 20 ft

STR

18 +4

DEX

8 -1

CON

16 +3

INT

5 -3

WIS

10 +0

CHA

5 -3

Condition Immunities: Blinded, Deafened, Exhaustion

Senses: Blindsight 50 ft (Blind beyond this radius), passive Perception 10

Challenge Rating: 5

Innocent Heart. Walter's infant corpse lies at the heart of the Flesh Mound, crying and afraid. If a player is Engulfed, they roll a d4. On a 2 or 4, they slip into a small demiplane, floating in darkness with only Walter in the flesh and gore. He has an AC of 10 and HP of 5. If he is reduced to 0 HP or is otherwise cleansed or calmed, the Flesh Mound dissolves, and his spirit is at peace.

Actions

Slam. Melee Weapon Attack: +7 to hit, Reach 5ft, one target. Hit: 13 (2d8 + 4) bludgeoning damage  When the Mound makes a slam attack and succeeds on hit, it has the option of grappling the target in exchange for doing damage. The target must succeed on a DC14 Athletics/Acrobatics check on its turn to escape the grapple.
Engulf. Melee Weapon Attack:+0. Hit: 2 (1d4) bludgeoning damage The flesh mound engulfs a Medium or smaller creature that is grappled by it. The engulfed target is restrained and unable to breathe. It must succeed on a DC14 Constitution saving throw at the start of each of its turn or take 1d4 bludgeoning damage. If the mound moves, the engulfed target moves with it. The mound can only have one creature engulfed at a time.

The Flesh Mound is not only Walter's soul, but the others who were sacrificed to this altar.

Beastial Huntsmen CR: 1

Medium humanoid, any

Armor Class: 13

Hit Points: 39 (6d8 + 12)

Speed: 30 ft

STR

16 +3

DEX

12 +1

CON

15 +2

INT

6 -2

WIS

12 +1

CHA

7 -2

Senses: passive Perception 10

Languages: Doesn't speak

Challenge Rating: 1

Keen Smell. The huntsman has advantage on Wisdom (Perception) checks that rely on smell.
Altered. The huntsman has advantage on saving throws against effects that target humanoids.
Brute. A melee weapon deals one extra die of its damage when the huntsman hits with it (included in the attack).

Actions

Attack. The huntsman makes one attack with each weapon. It has two weapons as detailed below. If it has a spear, it makes two spear attacks.
Saw. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 9 (2d6+2) slashing damage.
Torch. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 6 (2d4+2) fire damage.
Plow. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 13 (2d10+2) piercing damage.
Spear (Two-Handed). Melee Weapon Attack: +4 to hit, reach 10 ft., one target. Hit: 11 (2d8+2) piercing damage.

A specter of Canis's will that can be summoned in various ways.

Type

House, Large