Faerzress

Faerzress was a magical radiation only found in the Underdark, a remnant of the forces that shaped the Underdark itself. It interfered with the casting of teleportation and divination spells.

Etymology
The word faerzress is composed of faer (magic) and Z'ress (to hold dominance or to remain in force), which together mean magic that remained. However, its original term had faer and Z'ress inverted, which rendered the term as dominating magic. That older composition was much more telling about the primary task of these emanations: to keep the drow in the Underdark.

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.

After 1375 DR, drow wizards and clerics noticed sparks of Faerie fire when casting a divination spell. After some investigation, the combined efforts of the wizards of Sshamath and the priestesses of Eilistraee uncovered the plan of the clerics of Kiaransalee to open a gate to the Negative Energy plane by agglutinating a massive amount of Voidstone, which was augmenting the faerzress. However, it was deduced that it was only affecting drow spellcasters because they had faerzress in them, which was augmented and rendered unstable by the actions of the clerics of Kiaransalee. Thus, when they used divination, they expunged part of this excess of faerzress to the point where using divination would only render a vision of faerie fire.

Through the use of a Selu'Kiira, Q'arlynd Melarn was led to believe that, in ancient times, the forces of Aryvandaar not only wiped out Miyeritar, but also changed the bodies of the Illythiri, and created the faerzress to imprison them under the earth (both by preventing them from escaping by magical means, and by making them less willing to return to the surface, as the faerzress appeared to exert a calling effect on the drow). However, the Faerzress was actually far older than the Descent of the drow, and already existed when the events shown by the Kiira took place (as it dated back to the formation of the Underdark itself). Furthermore, the bodies of the Ilythiiri weren't altered by the Aryvandaari, but by Corellon's magic, as directed through his priests and High Mages from all Faerun, reunited at the Elven Court.

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.