Well, here we go. Note that killing the Slender Man may not kill it, but instead banishes it back to its point of origin. In such a case, further steps would need to be taken to permanently destroy the creature — there's a long-standing rumor that the Slender Man is some manner of psychic entity from the Astral Plane, and must be confronted on that plane to be permanently slain.
Unsurprisingly, this version of the Slender Man owes a lot of its lineage to Marble Hornets' Operator. Like the Operator, this Slender Man is probably best used as an antagonist when the (low-level) PCs stumble across its lair or cultists. It then performs weird, quick, blitz-style attacks, appearing at inopportune and seemingly random times to menace the PCs. It usually aims to frighten or wound rather than kill, reserving murder for anything that seems truly capable of threatening it. (Such as high-level PCs that come poking around its lair to put an end to it once and for all.)
The Dungeon Master probably wants to determine its agenda, if any. It might have a goal that drives it (the fact that it occasionally dominates cultists suggests it seeks worship or validation somehow), or maybe it's just a hyper-intelligent beast that feeds on the fear its presence causes. (Maybe it's even a psychic entity that requires minds to think of it, lest it winks out of existence. It would certainly explain why areas and people tainted with its influence count as "associated objects" when it uses its teleport power.)
And at least a couple of cagers have suggested its ability to maze people, its penchant for dismemberment, its complete silence, and its odd locomotion suggest kinship with the Lady of Pain somehow...
The Slender Man
Medium monstrosity (plant), neutral evil
Armor Class 18
Hit Points 187 (25d8+75)
Speed 0 ft., teleport 30 ft.
Str 16 (+3), Dex 22 (+6), Con 16 (+3), Int 22 (+6), Wis 19 (+4), Cha 17 (+3)
Saving Throws Dex +11, Int +11
Skills Insight +9, Intimidation +8, Perception +9, Stealth +11
Damage Resistances bludgeoning, piercing, and slashing from nonmagical weapons
Senses blindsight 120 ft., tremorsense 120 ft., passive Perception 19
Languages telepathy 120 ft.
Challenge 13 (10,000 XP)
Grappler. The Slender Man has advantage on attack rolls against any creature grabbed by it.
Nonterrene Consumption. The Slender Man does not require air, food, drink, or sleep.
Shapechanger. The Slender Man can alter its body proportions at will, extruding appendages or altering its size to Small or Large.
Sigma Radiation. Any humanoid that starts its turn within 30 feet of the Slender Man must make a DC 19 Constitution saving throw. On a failed save, the creature is poisoned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, with disadvantage if the Slender Man is still in its presence, ending the effect on itself on a success. If a creature's saving throw is successful or the effect ends for it, the creature is immune to the Slender Man's Sigma Radiation for the next 24 hours.
Teleportation. Three times per day, the Slender Man can innately teleport as per the spell (PHB, pg. 281), requiring no components. If the Slender Man is teleporting to its lair or to the location of someone it has encountered and is stalking, it is considered to have an associated object with regard to its destination. (100% chance of teleporting on-target.)
Multiattack. The Slender Man makes three tendril attacks.
Tendril. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 14 (2d8+6) bludgeoning damage, and the target is grappled (escape DC 19). Until this grapple ends, the creature is restrained.
The Slender Man can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Slender Man regains spent legendary actions at the start of its turn.
Missing Time. A targeted creature must succeed on a DC 19 Wisdom saving throw, or else take 13 (3d8) damage and become removed from time for 1 minute. While removed from time, a creature disappears and is incapacitated. A creature can repeat the saving throw, with disadvantage, at the end of each of its turns, ending the effect on itself on a success.
Slender Sickness. One creature poisoned by the Slender Man must succeed on a DC 19 Constitution saving throw or else take 13 (3d8) damage, fall prone, and become incapacitated for 1 minute. This effect ends if the creature is no longer poisoned.
The Slender Man's Lair
The Slender Man's lair is usually either an abandoned ruin made by humanoid hands, or a wooded area away from civilization. (Frequently, however, this wooded area is poorly-traveled, but not so remote as to be unreachable.) Even in woods, however, it frequently lairs among the detritus of civilization — an abandoned shack, a crumbling tower, a ruined fort. The Slender Man encountered in its lair has a challenge rating of 15 (13,000 XP).Lair Actions
When fighting inside its lair, the Slender Man can invoke the ambient magic to take lair actions. On initiative count 20 (losing initiative ties), the beholder can take one lair action to cause one of the following effects:
- The Slender Man targets any creatures that can see it within 120 feet of it with a fear effect. Affected targets must succeed on a DC 19 Wisdom saving throw or be frightened by the Slender Man. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. The Slender Man can't use this lair action again until it has used a different one.
- The Slender Man targets one creature that can see it within 120 feet of it with a mind-influencing effect. The target must succeed on a DC 19 Wisdom saving throw or be charmed by the Slender Man. The charmed target regards the Slender Man as a potent being capable of granting great enlightenment or power, and is inclined to assist it to further curry its favor. If the Slender Man attempts to take action against the target, it can repeat the saving throw with advantage. The charm effect lasts 24 hours, or until the Slender Man is banished, is destroyed, is on a different plane of existence than the target, or takes a bonus action to end the effect. The Slender Man cannot use this lair action again until the charmed condition is ended on the initial target.
- The Slender Man casts maze (no components needed; PHB, pg. 258-259) on one target that can see it within 120 feet of it. While maintaining concentration on this effect, the Slender Man can't take other lair actions.
The region surrounding the Slender Man's lair is warped by the creature's presence, which creates one or more of the following effects:
- Trees within 1 mile of the Slender Man's lair become twisted and spindly.
- Buildings within 1 mile of the Slender Man's lair become increasingly decrepit, and are frequently vandalized.
- Creatures within 1 mile of the Slender Man's lair make saving throws against the frightened effect at disadvantage.
If the Slender Man dies or is banished, these effects fade over the course of a year.