Faerzress was a magical radiation only found in the Underdark, a remnant of ancient Spellcraft that helped shape the Underdark itself. It interfered with the casting of teleportation and divination Schools of Magic. Areas of high faerzress concentration would manifest physical signals, ranging from subtle shifts in temperature to visibly glowing rock formations. Many plants and fungi in the underdark found ways to live on a diet of faerzress, such as Bluecap.
The word faerzress was composed of the Dark Elf words faer (magic) and Z'ress (to hold dominance or to remain in force), which together mean "magic that remained". Drow sought out areas with strong faerzress to build their cities and outposts in order to keep others from scrying their doings or teleporting inside the city. Drow also used faerzress to create magical artifacts and sustenance.
Faerzress could affect magic and spellcasting in unpredictable ways, in some ways like Wild Magic. It was known to mainly affect divination and conjuration spells, making teleporting by magic in the Underdark a risky effort. It was possible to enact rituals to alter the concentration of faerzress in an area or to affect a pocket of it, although such spells often attracted unwanted attention from denizens of the Underdark attuned to the faerzress.