Many parts of the Underdark of Faerun are suffused with a magical radiation that the drow call Faerzress. A remnant of the mighty forces tha originally shaped the terrain of the Underdark, faerzress distorts and interferes with certain types of magic.
The effects of faerzress on spells are not widely known among surface-dwelling characters. Before a spellcaster attempts a spell that would be affected by the Underdark's magical conditions, a DM may allow the character a
DC 25 Knowledge (arcana) or Knowledge (Underdark - local) check to determine whether he or she happens to know about the hazards.
Areas of faerzress can be found here and there throughout the Underdark. The drow have long sought out places of powerful faerzress in which to raise their cities, since the magical radiation helps to frustrate spying and protect against enemy assaults. The magical energy can also be used in the construction of various powerful magic items.
Faerzress has the following effects.
Divinations: every creature in an area affected by faerzress receives a +4 bonus on Will saves against all divination spells,
including scrying and greater scrying.
DB Note Divinations also have a 25% chance of failing each and everytime cast in an area of faerzress, or when attempting to affect someone or something in an area of faerzress.
Spells of the conjuration (teleportation) subschool do not work reliably over distances greater than 1 mile when either the origin or destination is within an area affected by faerzress. DB Note: It is the origin and destination that is important here, not the distance.
A character who casts a teleport spell or uses an ability or item that duplicates that effect must make a DC 35 Spellcraft check. Success indicates that the spell works properly; failure means that the teleporting character automatically suffers a mishap, just as if he or she had rolled 100 on the table in the teleport spell description, followed by an "off target" result.
A character using a normally infallible form of teleportation magic (such as word of recall or greater teleport must also make a DC 25 Spellcraft check. Again, success indicates that the spell works properly, and failure means the character suffers a mishap and an off-target result, as described above.