Consider a haunted house. There is the sense of the place itself acting against intruders and supporting its own allies; doors slamming, windows becoming gates to other worlds, messages on mirrors. Maybe ALL WORK AND NO PLAY MAKES JACK A DULL BOY, and characters enter the madness of the place; perhaps apparitions show themselves and characters are misled in that way. Perhaps objects animate, or cold spots enervate, or someone is crushed by a falling chandelier.
Some of this is due to the haunt’s association with a whole pack of ghosts. It seems that the tragedies of such a location tend to accrue, and where originally it was simply a violated burial ground, it swiftly becomes a site of a grisly murder, and then perhaps a tragedy murder-suicide, and then perhaps a serial-killer’s staking ground. By the time the latest victim arrives, it’s a wonder they can find a spare bedroom amongst the ghostly throng!
The tools to enable this are simple: the possession ability of ghosts is our baseline. I might replace the ghost’s ghastly visage with a dreadful moan (substituting a mummy’s paralysis for the ghost’s aging effect, and sound for sight) in such a world, since the ghost will never be seen (it’s a haunter). Of course, we will also use specters and their poltergeist variant heavily. The ghost is the centerpoint, though: it can move from room to room, and possess objects (causing them to animate per animate object). When it animates the room itself, it can slam apertures and fling small objects, but it still has to more or less obey the logic of the structure.
Such a ghost will need lair actions, of course; that’s where the rest of the supernatural effects will come from.
Medium undead, any alignment
Armor Class 14 (magical armor)
Hit Points 72 (16d8)
Speed 0 ft., fly 40 ft. It can hover.
STR(-2) DEX(+1) CON(+0) INT(+0) WIS(+1) CHA(+3)
Damage Resistances acid, fire, lightning, thunder, bludgeoning, piercing, and slashing from nonmagical weapons
Damage Immunities cold, necrotic, poison
Condition Immunities charmed, exhaustion, frightened, grappled, paralyzed, petrified, poisoned, prone, restrained
Senses darkvision 60 ft., passive Perception 11
Languages any languages it knew in life
Challenge 8 (3,900 XP) (9 in lair; 5,000 XP)
Ethereal Sight. The ghost can see 60 ft. into the Ethereal Plane when it is on the Material Plane, and vice versa.
Incorporeal Movement. The ghost can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.
Invisibility. The ghost is invisible.
Rejuvenation. As long as the ghost’s lair still exists, it will reappear in the lair 24 hours after being destroyed. This can be prevented via barring extradimensional access to the lair (such as via forbiddance or hallow), by resolving the ghost’s reason for continued existence, or by substantially destroying the lair in the ghost’s absence.
Legendary Resistance (3/Day). If the ghost fails a saving throw, it can choose to succeed instead.
Withering Touch. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit: 17 (4d6 + 3) necrotic damage.
Etherealness. The ghost enters the Ethereal Plane from the Material Plane, or vice versa.
Possession (Recharge 6). One humanoid that the ghost can see within 5 ft. of it must succeed on a DC 14 Charisma saving throw or be possessed by the ghost; the ghost then disappears, and the target is incapacitated and loses control of its body. The ghost now controls the body but doesn’t deprive the target of awareness. The ghost can’t be targeted by any attack, spell, or other effect, except ones that turn undead, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to being charmed and frightened. It otherwise uses the possessed target’s statistics, but doesn’t gain access to the target’s knowledge, class features, or proficiencies.
The possession lasts until the body drops to 0 hit points, the ghost ends it as a bonus action, or the ghost is turned or forced out by an effect like the dispel evil and good spell. When the possession ends, the ghost reappears in an unoccupied space within 5 ft. of the body. The target is immune to this ghost’s Possession for 24 hours after succeeding on the saving throw or after the possession ends.
The ghost can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature’s turn. The ghost regains spent legendary actions at the start of its turn.
Withering Touch (2 actions, not while possessing). The ghost uses Withering Touch.
Etherealness (not while possessing). As long as the ghost is not in (or entering) an area of daylight, the ghost uses Etherealness.
Melee attack (only while possessing). The ghost makes a melee attack.
Glide. The ghost moves its speed without provoking opportunity attacks.
On initiative count 20 (losing initiative ties), the ghost takes a lair action to cause one of the following effects; the ghost can’t use the same effect two rounds in a row:
- A magical mournful wail fills the lair. All creatures within 60 feet of the ghost who can hear the wail must succeed on a DC 14 Wisdom saving throw or be frightened of the lair for 1 minute and flee it. A frightened creature can repeat this saving throw at the end of each of their turns, with disadvantage if they can still see the lair. After a creature is no longer frightened by this effect, they re immune to it for 24 hours.
- Objects in the area magically animate. The ghost casts animate objects with no components and with a duration of 1 minute (instead of concentration), and with only 7 creatures-worth of value (instead of 10). If this ability is used for a second time, the previous casting ends.
- The area is filled with images important to the ghost. The ghost casts major image with no components and with a duration of 10 minutes (instead of concentration). If this ability is used for a second time, any previous casting ends.
The region containing a legendary ghost haunt’s lair is warped by the ghost’s magic, which creates one or more of the following effects:
- Bad weather is common within 6 miles of the haunted site, with impossible weather phenomena (stationary spiraling clouds) common.
- Those who sleep within 1 mile of the haunted site have dreams shaped by the ghost; spell slots cannot be regained at the end of a long rest within that area unless the ghost allows it.
- The violence and tragedy which surrounds the ghost weakens the Material Plane. Fissures to the lands of the dead or to fiendish planes allow creatures native to those places to dwell nearby.
If the ghost is destroyed, these effects fade over the course of 1d10 days.
Lemmee just say this: Legendary creatures’ stat blocks are a pain in the rump. That is all.