This is pretty much my take on Dark Souls invasions in D&D mashed up with The Nightmares Underneath
There is a spell, The Happy Hunt, that comes and goes in waves of cultish aristocratic fancy. "I am so bored" a foppish noble youth exclaims in some den of decadence, and some figure or another smirks and says, "Well, I have something fun in mind you've yet to try, I guarantee" and after some simple rituals, they dream of going off to dream of murdering someone and take a bit of their soul if successful, and the rumors and dreams of murder spread and spread until eventually the cult grows too large and self destructs as they attack each others nightmare realms over imagined and real wrongs, or some moral authority shuts the whole thing down via torch and pitchfork. But copies of the ritual always escape or are dug up elsewhere or revealed unto mad wizards in swirling and churning nightmares.
The Happy Hunt
Target- One individual, who can be specified by name. If they are more than 10 miles away, part of their body must be in the possession of the caster, and it must be eaten. Alternately, random targets can be chosen.
Duration- [level] or [sum] hours or until the target is slain. The caster will remain asleep for the duration.
Range- 10 miles or Infinite(provided requirements are met)
Effect-The caster falls into slumber for the duration, and their psyche astrally projects to appear somewhere near their target... or so it seems. The astral projection is clearly some sort of magical entity, but it is recognizable as the caster and functions exactly as the caster would, with a few exceptions.
If the caster's projection is killed, they simply wake up.
If the caster's target is killed, they ALSO simply wake up, the events of the day being retconned into a bad dream.
However, successfully killing the target drains them of a level (or HD) of experience, and grants the Happy Hunter XP for the combat. Variants may allow other things to be stolen- spells, secrets, statistics, even the dream-versions of magic items, but the loss of level from the target is a given- the psychic trauma of being 'slain' and having bits of ones soul pillaged is extreme.
If the target is killed enough times to reduce their HD or level to 0, they are truly slain, and their body may be possessed by the offending Happy Hunter. Furthermore, the displaced and damaged soul of the slain will likely persist as a Dreamwraith.
Dreamwraiths- Not true undead, these are souls that linger in the dream realm, delaying whatever final destination their souls are drawn to. Though able to interact only with dreaming mortals typically, one salient point to make is that Dreamwraiths are perfectly capable of casting The Happy Hunt and menacing the waking world even when their bodies are long dead. However, Dreamwraiths still need an anchor to the living world, which will typically end up as the skull of their old body. Without such an anchor, they have no focal point from which to stage Hunts and will be unable to further menace the world.
Powerful Dreamwraiths can create mental palaces in the dream realm where they store pillaged and imagined dreamstuffs to create a more pleasant abode for themselves. This is an alluring alternative to any promised afterlife, as successful Happy Hunters could begin enjoying such psychic demenses even while still alive. In The Nightmares Underneath terms, these palaces would be Nightmare Realms, with the skull of the Dreamwraith/Happy Hunter serving as the Anchor.
- A long-dead shaman lies somewhere in a Mound, his dreamwraith hunting those to sustain its existence in Dream and slaying those who attempt to find his hidden body.
- In the distant past, there was a kingdom that delved deeply into the Happy Hunt and while the dreaming knights and lords and ladies fell into ruin in reality, in the dream realm they live on in immortal palaces of fancy, occasionally devouring the minds of those who trespass their ruined castles and estates. Still, that only means there's a crumbling palace in reality AND in dream to loot!
- A serial killer uses this spell to kill without fear of reprisal. In a city of thousands of people, the reward for his capture is stupendous, but how to find him?
- The spell has grown popular among drug-addled nobles, who hire outside help to assist construction of their fanciful dream-realms and to guard them from rival Hunters.
- A plot to kill the Emperor with this spell has been uncovered, and hysterical witch-hunts wrack the land.
- A dead past PC or NPC has become a Dream Wraith, and targets the players out of vengeance or madness. Can they be reasoned with, or is a trek back to where their body is a necessity? Hopefully it's not like, at the bottom of the ocean or something.