|It was going to be this or Fuseli's The Nightmare, wasn't it?|
Armour Class Unarmoured Hit Dice ½ Move 50% Normal Human Morale 6
Neutral (evil) with Unratable Intelligence
An oneiric phobophage, trolling the borders of sleep.
Not every sleep paralysis episode is a demon, and it’s not necessarily a demon.
It can be a man with a hat, a hooded figure, a thing without thumbs, a ghoul, a ghost, a goblin, the incubus-succubus, a recently dead relative, your double, someone else's double, a dolphin transformed into a handsome young man, the Old Hag, a coiled snake, a house cat, a caterpillar, a spider as big as a pony, wings beating against your face and chest, an old woman combing her long shaggy black hair, a dead person on you, a little jewel-eyed idol, an enormous dog or cat, a demon lover, your favourite childhood toy, a side of beef, an astral body, a glistening blood red horse, a statue, a mossy tree trunk, a gargantuan pig, a toad, a tombstone, a single piece of straw, a wheezing voice calling for "more weight", nothing you can see.
It has a thousand forms, appearing across cultures and history.
By day, it barely exists - a thing without shape, sapience or senses. It cannot be detected except by extraordinary means. Second sight glimpses it, but makes no sense of what was seen. The demon wanders without trace or volition.
As night falls, it gravitates towards intelligent beings as they settle down to sleep. Second sight may catch it, mistaking it for a small animal slinking through the shadows - if the demon is surprised, the form that it shows its victim will be seen (save vs. fear as a watcher, below).
Randomise/pick a PC/NPC as a subject, likewise the hour of the demon's visit. It will come when the subject is alone or asleep or when everyone else is asleep/suitably diverted.
Under ordinary circumstances, only the victim can see, hear and feel the demon, and must make three saves against sleep paralysis per visit.
Three successes or a single crit. Paralysis is broken and the demon vanquished; it will not prey on this character again.
One fail. You have a horrible night. The demon feeds and it as if you have had no rest at all - you are Fatigued as well as any other consequences of losing a night's sleep.
Two fails. Fatigued as above, and the demon will come back again tonight for another go.
Three fails or a single crit. The demon latches on, visiting you night after night. You make saves vs. charm, disease, fear and illusion at disadvantage, in addition to losing sleep. Thereafter, only a critical success or other means will vanquish the demon - three successes only ensures your Fatigue doesn't get worse.
Fatigue is cumulative: Exhausted, then Spent, and then you make death saves each night to see if you wake up at all. While under the influence of the demon, you are in an Altered State.
Sleeping in a protection from evil circle (or an equivalent effective traditional folk remedy) allows you to get an undisturbed rest and reduce your Fatigue, but does not break any link between you and the demon.
Having someone watch over and wake you from paralysis episodes will prevent your Fatigue worsening - if they're prompt, you won't have to risk any saves.
The watcher must roll to see if they are surprised by the demon. If they are, they will catch a glimpse of it and must save vs. fear or be frightened - all ability checks and attacks at disadvantage while watching over the sleeper thereafter. On a critical success the demon is vanquished; on a critical fail the demon will visit the watcher next time they sleep, and they will make their saves at disadvantage.
Using detect evil or detect invisible carries the same risk as being surprised.
Other than withdrawing, the demon's only defence is to make a fright attack. As above, but a crit success does not vanquish. It may make one per round.
It is immune to non-magical damage, except where appropriate for an effective folk remedy. A bless spell vanquishes the demon, no save.
Did you ever have sleep paralysis? I have - with the murky dark-on-dark shapes and the pressing feeling and the desperation to make a sound, twitch a limb to break it.
These mechanics can serve for mundane sleep paralysis and/or SADS, if you don't want them attached to a monster. The spells mentioned probably don't do you any good, in that case.
Unless the demon latches on to you, I think you can probably get away from it by sleeping somewhere else. This could be as easy as changing rooms (its senses are rudimentary, its intelligence negligible) or as significant as leaving the region/country.
And there's not sleeping on your back, too.
There are numerous traditions that the demon has a hat you can grab and pull out of the dreaming. Wouldn't that be a thing to see?
The dreaming may also touch the Astral/Ethereal/Spirit Plane(s), depending on the setting cosmology, so the sleep paralysis demon may have another form or be the minion of something that dwells there.
While it could be a pushover for your traditional fantasy adventurers, it's almost impossible to detect or fight if you're nearer the Normal Human end of the spectrum. It's not meant to be a combat monster, but to make your life miserable until you can find some way of dealing with it other than hoping for good dice rolls. If you're all willing to play the fiction and the mechanics, you can even pretend that there's no monster threat until properly diagnosed.
In another iteration it doesn't have to be a monster in its own right, just some of the abilities of a ghost/haunting - like the Walker Alone.
Owes a debt to the Nightmare from Dragonwarriors Book 4 and the Dreamslayer from Spelljammer (for crits on saves, too).