The Abyssal Situation
According to the Rule of Threes, there are three things you have to be particularly careful about, when you cast spells on the Abyss. First of all, the Abyss very easily corrupts Alteration magic. Whenever an arcane caster attempts a spell of this school, he or she has to make a Spellcraft check, DC 15 + two times the spell level. If the check succeeds, the spell works as normal. If it fails, though, the spell is corrupted in some way. The DM is free to decide the exact result, but as a general guideline, a spell that doesn.t physically change the target shouldn’t become directly dangerous (for example, fly could cause the target to sprout wings, and cat’s grace could cause the target’s eyes to become catlike). A spell that actually changes the target’s body, instead, could cause dire and even permanent consequences (stone skin could actually turn the target to stone).Secondly, magic on the Abyss tends to draw unwanted attention, more than in any other place.
Don’t spy on fiends, don’t call on fiends, keep your head low. Whenever you cast a Divination spell, the local demon lord will know it, and will be able to strike at you using the power of the Abyss. The lord can send a spell at you through your divination, provided that the spell is something related to the divination itself (for example, discern lies could be the conduit for a suggestion, but not for a daze spell), or he could send you false information. It should be noted that this requires the direct attention of the demon lord; most of them will not bother with low-level, frequently used divinations. Detect thoughts and similar spells or effects are not to be used on tanar.ri; attempts to do this force the caster to roll a Will save, DC equal to 15 + the demon.s Charisma, or suffer permanent insanity of a type decided by the DM (heal, restoration, wish, limited wish or miracle can restore the character.s mind). Success means that the caster takes 1d4 Wisdom and 1d4 Intelligence damage, and may remember fragments of information. Callings tend to be intercepted and turn up a demon, too. The chance for this is 10% per spell level; the higher the spell, the more powerful the fiend.
Necromancy has a few problems; minor demonic spirits generally inhabit freshly created zombies or skeletons, and this usually results in the undead attacking their creator.
Reincarnation always causes the target to assume some kind of demonic form. A good character that dies in the Abyss without receiving a blessed burial spell will return as a bodak.
There are a couple benefits too, if you like chaos and destruction. Damage-dealing evocation and
conjuration spells are automatically empowered, as per the Empower Spell metamagic feat.
Illusions are automatically extended and the DC for any Illusion spell is increased by +2.
Evocation and conjuration spells that deal damage are automatically empowered, as per the Empower Spell metamagic feat.