Faerzress

Faerzress is a magical radiation only found in the Underdark. It is said to be a remnant of the forces that shape the Underdark terrain and it interferes with the casting of some spells. Although the knowledge of faerzress among surface-dwellers is sparse, it is known to mainly affect divination and conjuration spells making teleporting by magic in the Underdark a risky effort.

Casting spells
A caster who want to cast such a spell affected by faerzress can choose to make a DC 25 knowledge (arcana) or knowledge (underdark) check to determine the hazards of casting that particular spell.

Every subject to a divination spell in an area affected by faerzress receives a +4 bonus on Will saves against all divination spells, including scrying and greater scrying spells.

Casting a spell from the conjuration subschool such as teleportation over more than one mile when either the origin or destination is in an area affected by faerzress proves to be unrealiable. A caster must make a DC 35 Spellcraft check to determine a succesful teleportation when using a spell or magical artifact to teleport. Failing the check means teleporting 'off-target', which can result in a deadly teleport to the inside of a rock or ending up being teleported above a deep underground chasm.

Faerzress and drow culture
Faerzress is a drow word, and plays a major role in drow city planning. Drow search out areas with strong faerzress to built their cities and outposts in to keep others from scrying their doings or teleporting inside the city. Drow also use faerzress to create magical artifacts.

Effects on plantlife
Since the underdark lacks food or light for plant life and funghi, many plants and funghi found underground have found ways to sustain of a diet of faerzress.