Faerzress

Faerzress was a magical radiation only found in the Underdark. It was said to be a remnant of the forces that shaped the Underdark terrain and it interfered with the casting of some spells.

Faerzress and drow culture
Faerzress was a Drow word, and it played a major role in drow city planning. Drow sought out areas with strong faerzress to build their cities and outposts in to keep others from scrying their doings or teleporting inside the city. Drow also used faerzress to create magical artifacts.

Rumor said that faerzress was in fact created by Elven High Magic when the drow retreated to the Underdark so they could not escape by magical means.

Effects on magic
Faerzress could affect magic and spellcasting in unpredictable ways, in some ways like wild magic. Although the knowledge of faerzress among surface-dwellers was sparse, it was known to mainly affect divination and conjuration spells, making teleporting by magic in the Underdark a risky effort.

Some areas in the Underdark had stronger faerzress emanations than others. For example, Zz'Pzora's Lair outside of Menzoberranzan had very strong emanations which Liriel Baenre was able to capture in the Windwalker amulet to carry drow magic to the surface.

Effects on plantlife
Since the Underdark lacked food or light for plant life and fungi, many plants and fungi that are found underground found ways to live on a diet of faerzress.