Incendiary Entrapment

This spell allows a wizard to create a magical trap upon a floor, stairway, door, or other surface. The area affected is 100 square feet at 14th level, with an additional 50 square feet per level affected thereafter (thus, a 20th-level wizard can affect a 20 x 20-foot area).

When a creature touches the surface, the area explodes into flames after a delay of 20 to 60 seconds, causing combustibles to ignite and other substances to melt, vaporize, or char, unless the item makes a saving throw vs. magical fire. The caster can specify the delay when completing the spell, but the delay cannot be changed thereafter. The flames inflict 1d4 points of damage per level of the caster upon all creatures within the area of effect. A successful saving throw vs. spell reduces the damage by half. The incendiary entrapment spell lasts until a creature contacts the surface and triggers the explosion. To complete this spell, the caster must sprinkle a mixture of finely powdered magnesium and saltpeter upon the surface to be enchanted.

Notes: Uncommon for Fire mages; otherwise, very rare. (Updated from Polyhedron Newszine.)