Arcane Magic May Not Behave
Magic is a force of nature, wild, subtle, and sometimes unpredictable. To reflect its inherently chaotic nature, arcane spells may not always work as intended; the more power the spellcaster draws the more likely something will go awry. To reflect this, every time a spell is cast (including from spell completion items, like scrolls, or spell trigger items, like wands and staves), there is a percentile chance equal to the square of one plus the spell’s level1 that the spell “does not behave as intended”. However, this possibility can be mitigated through knowledge and practice. As a swift action the caster may make a Spellcraft check2; each point by which the check result exceeds DC10 reduces the percentile chance described above by one. Time and care can also reduce this chance; each extra standard action spent casting the spell reduces the percentile chance by ten. In addition, specially prepared reagents can also decrease this chance. Regardless, spells have a minimum percentile chance equal to the spell’s level1; however, if the caster has a number of Spellcraft ranks equal to the base chance of the spell (after modifications), the spell will always work as intended.
Example: A 9th level mage casts the spell fly. As a 3nd level spell, its base percentile chance not to behave “as intended” is 16%. The mage makes his Spellcraft check for a result of 28. This will reduces the percentile chance to 3%. This is equal to the spell’s level, and is the minimum possible for him because he does not have 16 ranks in Spellcraft. (If the mage were 16th level, he would not even have to roll; he knows the magic so well that it will always behave “as intended”.) If, however, the mage cast fly with Silent and Still, the base percentile chance would be 36%, as the spell is now effectively 5th level. The same Spellcraft check of 28 would reduce the percentile chance to 8%. If the mage wished, he could spend an additional standard action casting the spell to reduce that chance to 5%.
The base percentile chance for Spell-like abilities is half of that for a similar spell; supernatural and extraordinary abilities that mimic spells are not subject to these rules.
1 This includes adjustments from metamagic feats. For the purpose of determining the percentile chance of “not behaving as intended”, spells affected by the line of “Sudden” feats (Sudden Widen, Sudden Extend, Sudden Empower, etc) add effective levels equal to the base feat plus two. So, to continue the example above, fly affected by Sudden Extend would effectively be 6th level (base of 3, + 1 for Extend, +2 for Sudden), and have a base percentile chance of 49%.
2 One may take 10 on this check if not hurried or distracted; one may never take 20.