Boogeyman

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny insect. The swarm can't regain hit points or gain temporary hit points.

Additionally, at the start of a creature's turn in which the swarm is occupying that creature's space, that creature takes 5 (2d4) poison damage. The swarm can make grapple checks against opponents, but cannot be grappled itself; when making grapple checks, it can use acrobatics instead of athletics.

Multiattack. The boogeyman may use its cthonic presence; then, it makes 3 bite attacks.

Bite. Melee Weapon Attack: +14 to hit, reach 5 ft., one target. Hit: 12 (2d4 + 7) piercing damage and 10 (4d4) poison damage.

Cthonic Presence. Each creature of the swarm’s choice that is within 60 feet of the swarm and aware of it must succeed on a DC 18 Wisdom saving throw or become frightened or charmed - the swarm's choice - for 1 minute. A creature can repeat the saving throw at the end of each of its turns, 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 swarm’s Cthonic Presence for the next 2 rounds.

A creature frightened or charmed by this effect is forced to see what the boogeyman chooses. This may be spectres of their worst fears come to life, particularly precarious obstacle courses, images of bugs squirming into their bodies, or just a distortion over everything around them. Either way, anyone effected has disadvantage on attack rolls and ability checks that are made to oppose the boogeyman.

The boogeyman can also choose one creature affected by this at the each of its turns; that creature perceives one of its allies as being the boogeyman instead. Attack rolls against a creature who is seen as the boogeyman, due to hallucinations, do not have disadvantage, as these checks do not run counter to the boogeyman's goals.

Squirm. The swarm can disengage as a bonus action.

Swarm (Mythic Phase Only). The swarm can grapple as a bonus action.

Psyche! Gotcha! When a hostile creature charmed or frightened by the boogeyman (as by its Cthonic Presence action) targets the boogeyman with a melee attack, the boogeyman can force them to re-direct the attack to another creature within range of the attack.

The swarm can take 2 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 swarm regains spent legendary actions at the start of its turn.

Bite. The swarm makes one bite attack.

Switcheroo (Costs 2 Actions). The swarm targets two creatures affected by its cthonic presence. Those creatures swap minds. This may be handled as the DM wishes; you may literally, physically swap the creature's character sheets; or the creatures may take each other's turns but have to verbally communicate over what is on their sheets without literally exchanging them; or players may still take their own turns, but attempt to act as the other creature would in their place. Whatever the case, this effect ends when either of the creatures succeeds on their save against cthonic presence, or otherwise has the cthonic presence's effect on them end.

If the boogeyman would be reduced to 0 hit points, his body spreads out, causing his form to grow into a massive, squirming mass of horror. His current hit point total and maximum is instead set to 300, he regains any expended uses of Legendary Resistance, and regains his Legendary Actions for that round - additionally, he now has 3 legendary actions per round instead of two. Once this transformation has occurred, the boogeyman can now use the options in the "Mythic Actions" section below for 1 hour. Its Mythic Actions are just further options for Legendary Actions.

Sting. The swarm targets one creature it has grappled with a bite attack. If that attack hits, the target must make a DC 16 Constitution saving throw or become poisoned.

Subsume (Costs 2 Actions). The swarm targets one creature it has grappled, and attempts to subsume that creature into its mass. The creature must make a DC 18 Strength saving throw, taking 35 (10d6) poison damage on a failure, but escaping the grapple on a success. If the target fails this saving throw, they become restrained by the swarm. A target restrained instead of grappled can still be targeted by any actions that specify a grappled target in the boogeyman's stat block.

If a creature is brought to 0 hit points by this attack, they are subsumed into the swarm's mass. A subsumed creature does not have to make death saving throws unless they are freed from their restraining, and does not fail death saves as a result of taking damage. Until the creature is freed, their body is puppetted by the swarm. A creature puppetted by the swarm acts according to the swarm's alignments and goals, but still takes their own turns - allowing the player to interpret for themselves how their character, when puppetted by the swarm, would act. (It is worth noting that the creature being puppetted is not technically dead or unconscious, but still making death saving throws - so, if the DM wants to allow the player to attempt fighting back against the will of the swarm, or if the player specifically requests being allowed to fight back, the DM may (at their behest) have the player make saving throws to try and act as they would prefer on their turn. Perhaps Charisma saving throws, as to force their own strength of personality or magic; perhaps Strength or Constitution saving throws, to move their own limbs instead of letting the swarm do it for them. Either way, the recommended DC for these saving throws is 18.)*

Nest (Costs 3 Actions). The swarm attempts to crawl inside the mouth of one creature within 5 feet. The creature must make a DC 18 Constitution saving throw. On a success, the creature takes 17 (5d6) piercing damage from the crawling and stinging, but the boogeyman does not successfully crawl inside them. On a failure, the creature takes 35 (10d6) poison damage, and falls prone. If the swarm is inside a creature, they have resistance to all damage, and the creature they're inside takes the same amount of damage that they do. The swarm cannot take actions while inside another creature, and can spend 5 feet of movement to leave that creature again.

At the beginning of any creature's turn for which they're filled by the swarm, they may make a DC 18 Constitution saving throw to throw the swarm up. On a failure, they are possessed by the swarm on their turn. A creature possessed by the swarm must act as per the swarm's alignment and goals. (Note: technically, since the swarm is fully possessing and controlling the creature, it would make more sense if the DM acted with the creature's stat block on their turn instead; however, it is recommended that the player be allowed to take these turns and just act as per their own interpretation of the swarm's will, as this is more fun for the player being possessed.)

*(These same notes about the subsume action's restrainment also apply to possession through the Nest action.)