Explosive Rune Field

Conjuration (Fire)
Level: Sorcerer 4, Wizard 4,
Components: V, S, M,
Casting Time: 1 standard action
Range: Medium (100 ft. + 10 ft./level)
Area: 20-ft.-radius burst
Duration: 1 round/level
Saving Throw: Reflex negates
Spell Resistance: No
Material Component: A piece of parchment scribed with runes. The runes must be drawn with silver-flecked ink (25 gp).
Source: Player's Handbook II

You infuse an area with arcane patterns that shift and slide along the ground. They seethe with arcane energy, and an occasional bolt of energy surges from them with a bright flash.

This spell creates a hazardous field of energy similar to that created by an explosive runes spell. Unlike that spell, the runes created by this spell detonate when they come into contact with a living creature other than the caster. Any creature that begins its turn in the spell's area must attempt a Reflex save.
On a failed save, the creature takes 4d6 points of fire damage. Moving through the spell's area is hazardous at best. Creatures that move at their normal speed must succeed on Reflex saves to avoid the spell's explosive effects. Creatures that move at half speed through the spell's area can avoid the damage just as if they had succeeded on their saves.