The Underdwellers

Those with evil intentions in their heart need to beware attracting the prying eyes of the Underdwellers.
They are creatures that are one with darkness. They lurk about in the dank, pitch-black caves beneath our very feet, snuffing out any light they can get their hands on. However, although they are foul and wicked, they are weak on their own, and they know it. They know that we are protected by the blessed light of the Pentide, and they fear us for it. They are driven to oppose the Pentide and cast the world into darkness once again, but they know that we, as their children, are too strong for them to sway on their own, so they deal in sneaky and underhanded tactics.
They strike at night, when they do not need to fear the light, and grab up anybody who strays too far from the safety of our communities. Beware of walking alone at night, especially if you don't have something to light your way, as you will become a prime target for them. So long as you live your life in darkness, they can take you at will and turn you into one of their own. However, even if you walk in the light, do not think that alone makes you safe.
Underdwellers are tied to darkness on a level far beyond physical. They are servants of the darkness, so they embody it in mind and spirit as well. They hold nothing but wickedness in their hearts and are so unified with that darkness that in its presence, they can move in daylight just as freely as in the darkest nights. This is why it is not enough to walk in the light. If they sense that darkness in your heart or spirit, if you turn your back on the light of the Pentide, you will never be safe.
And so, be careful children, never to let that darkness find root in yourself, or they will sniff it out and come for you.

Historical Basis

This myth is specifically used as a boogie-man story to scare children into misbehaving, but it is actually rooted in truth somewhat. The Underdwellers are a real group of people that live in the cave systems beneath the Shindus valley who are known to exit the caves at night to scavenge food and occasionally attack people or settlements. However, they power and malicious intent are greatly exaggerated here.