Brittleskin

Shining South (ShS)Updated Forgotten Realms (3.5)

Brittleskin is a D&D 3.5 spell (Transmutation) from ShS. On class lists for Druid (Level 3), Ranger (Level 3), Sorcerer (Level 3), and others. Range or area: Medium (100 ft. + 10 ft./level); Saving throw: Fortitude negates; Duration: 1 round/level; Casting time: 1 standard action.

School
Transmutation
Casting Time
1 standard action
Components
V, S
Range
Medium (100 ft. + 10 ft./level)
Target
One creature
Duration
1 round/level
Saving Throw
Fortitude negates
Spell Resistance
Yes

You cause a creature’s natural armor to crystallize and become brittle.
Whenever the affected creature is hit in combat, part of the crystallized hide cracks and shatters, dealing it additional lethal damage equal to its natural armor bonus.
Thus, if a creature with +3 natural armor is affected by this spell, it takes an extra 3 points of damage each time it is struck by a melee, ranged, or natural weapon.
The creature’s actual Armor Class does not change.