Such tales are nothing but silly superstition of course, but this one does perhaps accurately portray how dangerous and vile mana crystals can be. They are the scourge of human civilization all across the world. No matter where you travel you cannot escape the lingering threat of these deadly formations.
The most common type of crystal is rarely even identified as such. It is a black oily looking growth that is usually mistaken for rock, hence it’s name Blackstone. Luckily, this basic form of mana is inert and has no magical effects; it simply grows and spreads deep within the dark corners of the earth.
As it expands, Blackstone consumes the dirt and rock around itself to create long twisting tunnels. Eventually, Blackstone growths will run their course and old crystal formations will dissolve, leaving nothing but rock behind. This is how significant portions of the Dark Paths were probably created.
Unfortunately, that is not all Blackstone does. As it expands the crystals will occasionally consume something that causes what is commonly known as a mana stone to form. These crystals are bright and occasionally even glow, which makes them hard to miss. They come in many different colors, shapes, and sizes, but the important part is the magic that lies within them.
Every mana stone has a unique magical effect that a mage can produce by using their mage gift to burn the crystal. While many are useless or even dangerous to users, some are extremely powerful and useful.
These stone are the foundation of a mage's power, and all the stones created by the guilds are copies or offshoots of naturally formed mana crystals. This makes such stones incredibly valuable to mages, and thus treasures of great value. Many a life has been lost searching for such stones.
The Dark Paths are of course extremely dangerous themselves, but crystal formations make them even more deadly. For once you have Blackstone and mana crystals forming you will also start to see mutation.
Exposure to mana crystals sometimes causes the crystals to take root within living things. The process transforms an animal and creates a mutant. Such creatures are not only unnaturally strong, large, and swift, but the mana within them can actually imbue them with magical powers.
These horrors are the stuff of nightmares, and a plague upon all human civilization. As crystal caves grow they can breach the surface, and deposit their vile contents upon it. Thus, what was once considered safe civilized land can suddenly become wild and dangerous, as mutant abominations run wild across it.
Stories abound of such beasts, such as those of hard working farmers being slain by rats the size of dogs that can pass through walls or drive men mad with their whispers. Only, by removing the mana stones and sealing the tunnel can the nearby lands be made safe again.
Some of the creatures that dwell within crystal caves have no obvious earthly origin, but instead seemed formed completely of the crystals themselves. Slimes are the best known of these, but rumors persist of hulking creatures made of Blackstone that dwell within the deepest parts of crystal caves.
The magic derived from crystal caves is useful, but personally I do not believe it makes up for the threat such formations pose to humanity. They are just far too dangerous and unpredictable; you never know where one will appear, or what will crawl out of it.
Monsters of legend such as the Breshura Cyclops or the Wurm of Terringale were probably mutants as well, and the stories passed own about these monsters puts those of any modern mutant to shame. These creatures leveled cities and cast down empires. One day, I fear that something truly horrendous will again be born within the depths of these horrid places, and then all of humanity will weep and know the face of fear
-Farstidiuous Der Lanmount, Royal Gervaien Surveyor.