Tiny Horrors

Unlike most eldritch species, tiny horrors have an inability to telepathically speak Eldritch and do not have an eldritch presence. According to the Eldritch Classification Guide they are listed under Lesser Horrors, and are considered the weakest eldritch creatures. What tiny horrors do have is the ability to travel on the veil, making it so they vanish from the material world but do not completely re-enter the Aether. This ability drew the attention of many researchers who captured these little things to find out what gave them the ability to travel on the veil. The answer was their "skin", while not technically skin since they are made completely out of mana, the density and formation has made it so the veil does not fully recognize these creatures as a threat. So tiny horrors are sometimes poached for their skins to make Veil Walkers or veil research equipment.

Swarm of Tiny Horrors CR: 3

Medium swarm of tiny eldritch creatures, unaligned

Armor Class: 14

Hit Points: 42hp (8d8+6) 8d8+6

Speed: 0 ft , fly: 30 ft

STR

10 +0

DEX

18 +4

CON

11 +0

INT

1 -5

WIS

10 +0

CHA

4 -3

Damage Vulnerabilities: radiant

Damage Resistances: piercing, bludgeoning, slashing

Damage Immunities: psychic

Condition Immunities: charmed, frightened, grappled, paralyzed, petrified, prone, restrained, stunned

Senses: darkvision 60ft., passive Perception 10

Languages: None

Challenge Rating: 3

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 horror. The swarm can't regain hit points or gain temporary hit points.
Blurry Presence. Creatures have a disadvantage of perception checks relying on sight on this creature.
Eldritch Body. When directly touched by a creature, that creature takes 1d8 psychic damage. 
Veil Walk. As a bonus action the swarm can travel on the veil, disappearing from the material world. They can move through objects and creatures as if they were difficult terrain. This ends at the start of their next turn.

Actions

Bites: Melee Weapon Attack: +6 to hit, reach 0 ft., one creature in the swarms space. Hit: 10 (2d8) psychic damage, or 5 (1d8) psychic damage if the swarm has half it's hit points or fewer. The target must make a DC 10 intelligence saving throw, taking 14 (4d6) psychic damage on a failed save, or half as much on a successful one.

Although not considered intelligent tiny horrors have a tendency to create swarms that can overwhelm the average fighter.

Average Length

5 inches