A haunted house is a great format for a dungeon in D&D. The Death House, the intro-adventure for Curse of Strahd, is a great intro to the terrors to be found in that adventure. Likewise, the House of Lament, which serves as the sample starting adventure for Van Richten's Guide to Ravenloft, is another great spooky house.
But what if you wanted something a little... simpler? But also crazier?
What if the monster haunting the house was the house?
Enter the Haunting Revenant, one of the upcoming options in the new Monster Manual.
As part of the previews, we've actually got the full (I think) stat block for the Haunting Revenant.
So, assuming this image link doesn't break at some point, we can take a look at the various important details.
At CR 10, meaning 5,900 xp, by the new DMG's (at some point I'll just start calling it the DMG) let's figure out what level this would be a reasonable solo-monster encounter. If we assume a part of 4 player characters, that means 1475 xp per person. That makes this probably a Low difficulty encounter for level 9 characters, a Moderate (leaning low) encounter for level 8 characters, and a little north of the standard High difficulty encounter for level 7 characters.
Naturally, we could make it tougher by adding some other monsters - but I'd be somewhat hesitant to do so given the singular horror of a giant house on the attack. In other words, I'd probably try to toss it at a level 7 party. Solo monsters are typically a little easier because players can just focus all their damage on the monster, and with only a single monster, crowd-control spells can trivialize the monster.
However, let's look at that list of immunities: with immunity to Charm, Fear, Prone, Poisoned... it's pretty nasty.
Probably the biggest threat that the Haunting Revenant has is its Invitation ability. In a 60-foot cone, every target needs to make a DC 17 Charisma saving throw or get sucked inside the home. While this status doesn't actually damage them, it does make them way more vulnerable. First off, the house has advantage on attacks against creatures inside of it, and furthermore, it essentially auto-counterspells any spell cast inside.
And it's not easy to get out - you need to either kill the thing or cast Plane Shift. Even at high levels, that's an expensive price to pay. And with a DC 17 for both its Invitation and Haunted Zone features, basically anyone who isn't a Charisma-caster is going to have a very rough time actually saving agains these features.
Now, it's notable that getting sucked into the revenant with Invitation is not quite like other "swallow" abilities. While the target is trapped inside, they aren't restrained or blinded or even take damage.
And that, my friends, is where we could have a lot of fun as a DM.
My first thought regarding this monster was simply for the party to find some remote house out in the woods, only for a blazing light to ignite within and then a big old monster fight to ensue. But we could have so much more fun with it!
Instead, let's imagine that the party has been hired to investigate the disappearance of some local minor nobleman - a Baron or something. The Baron has some enemies, and was not a terribly popular aristocrat, perhaps having enacted some draconian laws that saw minor criminals punished with torture or death. The party discovers that the Baron traveled to some tiny hamlet out in the woods or at the top of a hill, with a letter that implies that they were going to blackmail them or something - a scandal that the Baron would only want to resolve in person.
The party goes to the site where the Baron was supposed to drop off the payment - poetically, it's the house of one of the people he had put to death for some trumped-up charge. Maybe it's a place where the family tried to help the executed person escape, and the Baron ordered them all killed for aiding a criminal. It's a place of shame and darkness, and people just abandoned the building rather than having someone else move in.
The party arrives, finding the Baron's personal carriage, but their loyal footman (who drove the carriage) and the Baron are nowhere to be found. The party approaches the house, only for the door to eerily swing open, and they blink, and anyone who failed the Charisma save is inside the house.
But the fight doesn't start immediately (or at least, it doesn't unless the party uses a feature like Divine Sense to realize the building is undead). Instead, the party actually has some time to explore the house. They find the footman's body bludgeoned by kitchen pots or a statuette in the hall. They find the Baron's body hanging from a noose in the attic.
And then they try to leave. The door won't open. The windows are suddenly boarded over, and no matter how hard they try to pry them open, they can't. Now, perhaps, they try to draw their weapons or cast a spell to blast the door open.
And that's when you call for Initiative.
For sure, don't force them inside if they're suspicious (rather, don't railroad them if they succeed on the saving throw). But the house, at least, should look for all appearances like a normal house unless they use some divination power to determine it's not. Those inside will see the house come to life, a terrifying face appearing in the hearth, or in a stained glass window, or even forming out of the wood boards like that illustration for the Mansion on Dutch Hill in The Dark Tower Book III, the Wastelands.
While not a legendary monster, this feels like a creature you'll really want to build up to - having people talking about a spooky house well before the party actually gets there.
Now, in terms of tactics:
For a DM, the haunting revenant kind of plays itself - try to sweep as many people into it as you can each turn with Invitation, and then pummel PCs inside with Object Slam.
For players, the Invitation ability is going to be tough to deal with - with a DC of 17, if you don't have proficiency in Charisma saving throws and aren't a Charisma-based character (I think the only class that gets Charisma save proficiency but doesn't use it for spells is the Cleric, though I could be wrong) you're going to have probably only a +2 at best to that save, meaning that you're probably going to have a 70% chance or more to fail the save.
It also means that, for example, a Paladin might find themselves the only one outside of the revenant, unable to lend their aura or other helpful abilities.
Casters going to have a rough time as well - essentially hit by a counterspell on every single spell they try to cast (even if they do so in a way that normally can't be countered, such as an Aberrant Sorcerer using Psionic Spells). (Actually, it's worse than counterspell, given that it will still waste your spell slot - yeah, even if you do have Plane Shift, you've to try very hard to actually successfully cast it).
So, honestly, a Barbarian or Monk who can just get inside and wail on the house from the inside might be your best bet - with an AC of 20, it's not going to be super-easy to hit, but these classes can at least handle getting hit frequently and absorb a lot of that damage. (+9 to hit with advantage will be tough even against pretty high-AC characters like Fighters and Paladins).
Now, this being a revenant, the party either needs to use Dispel Evil and Good to keep it from getting back to them (something they won't have until level 9 at the minimum) or they need to convince it to cool it.
However, revenants are one of the unusual types of undead that are not explicitly evil. In our previous scenario, maybe we need to convince the spirit to accept that they've succeeded in taking their revenge, and that justice has been served.
And, of course, the revenant could even be an ally. Invitation, notably, causes creatures inside to have Total Cover against everything outside of the revenant. It could be a last-ditch attempt for the revenant to save the party, maybe give them a couple rounds to heal up against the actual bad guys while the revenant tanks the damage - the revenant can always pick up a new body anyway.
Seriously, I think this is a really amazing new creature that I can't wait to throw at a level-appropriate party (frankly, I'd love to work it into my Innistrad chapter of my Ravnica campaign, though it would certainly require some friends to be a threat to what will be a group of 6 level 18 players at that point).
Indeed, this is the kind of monster that gives me a lot of hope for the new Monster Manual - that, with ten years of experience with 5E, the folks at WotC have figure out how to make some really compelling and interesting creatures to terrify your players with.