You can use fire magic to set things aflame.
Prerequisite: Able to cast 3 spells that deal fire damage
Benefit: You can cast spells that deal fire damage as “scorch spells,” which burn at slightly lower temperatures but last incrementally longer, giving them a chance to catch foes on fire. Only spells that allow a saving throw and deal fire damage can be scorch spells, and the save DC of a scorch spell is 1 lower than normal (because the weaker, slower fire is easier to dodge). When you cast a scorch spell, any target that fails its save against the spell catches on fire. Targets with Improved Evasion are immune to this extra effect, as are creatures immune to fire damage and some constructs and inanimate objects made from substances that don’t burn (at the GM’s discretion).
Targets that catch on fire take damage at the beginning of your turn every round until the flames are extinguished. This damage is based on the level of the scorch spell that ignited them. A 0- or 1st-level spell deals 1 point of damage each round. A 2nd- or 3rd- level spell deals 1d3 points of fire damage each round. A 4th- or 5th-level spell deals 1d4 points of fire damage a round. A 6th-level spell or higher deals 1d6 points of fire damage a round. A target burns until it takes a standard action to extinguish itself, it enters an area fire cannot survive in (such as jumping in a river), or an ally makes a Heal check with a DC of 15 + level of scorch spell.
Adventurer’s Handbook: Genius Guide Volume 1