( Sandstorm, p. 53)
Your fire spells are so hot that they can damage creatures that normally have resistance or immunity to fire.
A searing spell is so hot that it ignores the resistance to fire of creatures affected by the spell, and affected creatures with immunity to fire still take half damage. This feat can be applied only to spells with the fire descriptor. Creatures with the cold subtype take double damage from a searing spell. Creatures affected by a searing spell are still entitled to whatever saving throw the spell normally allows. A searing spell uses up a spell slot one level higher than the spell’s actual level.