A Fae Mist O'er Hangs the Ghostlight Fen

The Ghostlight Fen presents a feature common to this world, but a greater danger in this place, the substance the current human inhabitants often call "magic" but their ancient progenitors called "fae." In the parlance of the original human colonists fae is a system, perhaps even a network, that spans the entire planet and can manipulate matter and energy in accordance with the will of the user. The indigenous species are born knowing how to manipulate this system in various ways, but other can learn to control it. Control is the keyword, and the system is psychoactive and will respond to unconscious mind as easily as the conscious.

Indeed, theorists in ancient times speculated that the fae was a created rather than natural phenomena and the demons from the unconscious of its creators destroyed them, leaving only their creations (the ieldri and others) behind.

Fae permeates and surrounds the world, but in some places it collects and goes awry. Some of those bad places were caused by overstressing the system, as the ieldri sorcerers did in their desperate war against the ylthlaxu. Others may be places where it has just broken down with time. The Ghostlight Fen seems to be one of the former type.

This dysfunction manifests itself several ways, but most particularly: peculiars and visitants. Peculiars are small, discrete areas of reality distotions generated using these tables. Visitants are more pseudo-encounters of weirdness using these tables. The chance of coming across these in a given hex in the Fen per day is as follows:

Green Fen Hex: Peculiar - 20%, Visitant 5%
Pink Fen Hex: Peculiar - 60%, Visitant 30%

Spellcasters and Fae: All arcane spellcasters (not just sorcerers) are subject to something akin to a wild magic surge. After casting a 1st level or higher spell, a roll of a 1 on d20 requires a d100 roll on the table in the 5e PHB. In green hexes, this roll is only required for the first spell cast by an individual caster per hex. In the pink hexes it is required for the first spell of each spell level cast by an individual caster. Clerical casting is only affected in pink hexes and in the manner of green hexes for arcane casters.

Inspirations: The concept of the fae was inspired by C.S. Friedman's Coldfire Trilogy, but also borrows from the some of the rationalizations of magic in Hite's Trail of Cthulhu: Rough Magicks, details of  Forbidden Planet (1958), and Roadside Picnic.