Creatures of the Undrelm

Now, children, you better behave or the creatures from below will come and get you in the night

— Commonly heard warning amongst regular folk

The Undrelm

The Undrelm is a series of tunnels and caverns that supposedly exist a few miles beneath the world's surface. According to the legends, all sorts of monstrosities live and lurk down there. The Undrelm is not believed to exist for the vast majority of people. Only the very few learned and those that have somehow managed to find their way down and back out of there know that it is real. As such, the Undrelm is the nightmare fuel that adults use to scare the children.
For the Dwarves, Gnomes and Elves, all of whom have Undrelm cousins, the existence of Undrelm is a fact in their societies. The mythological status only really pertains to human culture. However, those 'dark' versions of their allied races do form part of the stories designed to scare the children.

Creatures

Undwarves

The Undwarves are the supposed Undrelm cousins of the Dwarves. They are depicted with greyed skin and even more stunted growth than the regular dwarves. However, they do apparently hold the power to enlarge themselves and turn invisible. There are all sorts of variations with different powers.

Undelves

With dark skin, not just greys but purple hues too with typically white hair. Like the Undwarves, they are regarded as cruel and vindictive in the stories.

Undnomes

The dark versions of the Gnomes, like a surprising number of creatures that dwell below, are supposedly capable of invisibility but not the enlargement like the Undwarves. Again, they are depicted with grey skin. The Undnomes are separated from the other two 'Und' races by the fact that the Undnomes are not regarded as evil.

Eatar'Seera

Humanoid creatures with tentacled mouths. These are particularly used to frighten children with threats they will come and take them away back to the Undrelm. The Eatar'Seera are so-called because they supposedly dine on the brains of their victims. Eatar'Seera translates from the Old Tongue to 'Eater of Minds'. They are notorious in the stories for stealing children as slaves.

A Translation for Dungeon Delvers & Dragon Slayers

Undrelm - The Underdark
Undwarf/ves - Duergar
Undelf/ves - Drow
Undnome/s - Svirfneblin
Eatar'Seera - Mind Flayer/Illithid