Additional Spellfire Rules
Ed Greenwood updated the information on Spellfire in a Polyhedron column about a year ago. It was part of an Everwinking Eye column on Errata.
The FR7 spellfire text shouldn't be interpreted as forcing a spellfire-wielding character to change class. Rather, in any givenadventure, experience is gained in the character's class only if no spellfire powers are used. If any spellfire is wielded, all experience points gained in the adventure go instead to the character's spellfire level (which uses the wizard XP table), and individual experience awards for the character's primary class (see page 48 of the 2nd Edition Dungeon Masters Guide) are lost. At the first level of spellfire ability (not primary character class level), absorption of magical energy (from spells of all sorts, breath weapons, gaze attacks, magic item discharges, and just about anything else) is involuntary: the character drains any magic with which he comes into contact, including useful magic and healing spells (only rest or non-magical healing can restore lost hit points to the character). Absorption is strictly voluntary at the second level of spellfire ability and above.
A character who exceeds his absorption limit (which is his Constitution score x10) involuntarily releases one level of energy about every six seconds (10 times per round), suffering 1d6 hp of damage each time, until the moment his total energy falls back into the 310xConstitution2 category (see FR7, page 50).
At 1st level, this involuntary release is uncontrolled, and can harm friends and valued things nearby (within five feet).
At 2nd level and above, the agonized spellfire-wielder can urge the release in one general direction, provided the character is free to act. There is intense burning pain, such as that suffered by Shandril when destroying Rauglothgor's lair. The wielder must save vs. paralyzation at -2. If this save fails, a spellfire wielder of any level 3leaks2 energy as a 1st level spellfire wielder until the total falls back to a controllable number. If the save succeeds, the wielder can use the release as an attack, and can opt to release all or any part of the excess energy in a single burst; however, each excess level of energy still inflicts 1d6 points of damage on the spellfire wielder no matter how it is released.
A 2nd level wielder attacking with an involuntary, but controlled, energy release suffers a -3 "to hit" penalty. If there are multiple bolts released during the round, the wielder can attack multiple targets, roll for each target separately.
At 3rd level, the attack roll is made at a -1 penalty. 
At 4th level, the attack roll is normal.
At 5th level and above, the attack is made at +1. It increases by +1 per spellfire-level thereafter.
Overloading a spellfire wielder invites a deadly counterattack. However, spellfire wielders seldom willingly overload themselves, save in very emotional, exceptional circumstances (such as avenging the death of a loved one), as the pain and risk are simply too great.
All spellfire attacks have line-of-sight range
     the wielder can hit anything he can see
     and does not suffer penalties for range,
     concealment, or cover.
In Realms campaigns, only the DM can decide if a character is going to manifest spellfire ability (which may occur at any time, triggered by contact with magic or a magically-powered or using being). This ability is hereditary, but also crops up at random, at the will of the gods. It is a form of "wild magic" that usually foretells great upheavals. In Shandril's case, the upheaval was The Time of Troubles. 
Spellfire is very rare; while it is not true that only one spellfire-wielder can exist in Faerun at a time, known (revealed) spellfire talents attract a lot of unwanted attention (as Shandril unwittingly did) and are very few and far between. DMs should never add a PC spellfire wielder to a campaign without a lot of forethought; its presence can too easily be a "campaign wrecker" in the hands of skilled (or merely malicious) players. On the other hand, when a lower beginning-level character joins a mid- to high-level party, giving the newcomer spellfire can be a good way to prevent the fledgling PC from being ignored, ordered about, or forced to run a gauntlet of too-dangerous challenges. 


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