Instant Minefield

You create several hidden mines throughout the area that explode when stepped on. Choose up to 6 unoccupied squares within range. A mine appears in each chosen square, visible only to you and your allies. A mine can't be targeted and isn't affected by area effects, but if it hasn't been triggered by the end of the spell's duration, it dissipates harmlessly. A creature that can see Invisible creatures and objects can see the mines. A creature can also attempt a Perception check against your spell DC to to find an invisible mine.

A creature who enters a square with an invisible mine triggers it, causing an explosion that deals 3d6 fire damage and 3d6 piercing damage (basic Reflex save) to the creature and every creature in a 5-foot area from the mine's square.

Heightened (+1) The fire damage and piercing damage dealt by a mine each increase by 1d6.