Caltrops

This spell allows a priest to plant a section of ground with magically created caltrops.
The spell can create two kinds of caltrops: infantry and cavalry. The first are of small size and are designed to harm foot soldiers. The latter are larger and cause serious damage to cavalry or units composed of size L or larger creatures. Cav- alry caltrops are so large that size M or smaller creatures can easily step around them. This pre- vents damage to infantry units.
Each time a unit moves into a planted area, the unit suffers an attack of AD = 4 (for infantry caltrops) or AD =6 (for cavalry caltrops). Units charging through a planted area suffer double damage. If a unit ends its movement in a caltrop-sown region, it suffers another attack when it moves out of the area.
This spell can create a rectangular field of infantry caltrops up to 160 square yards in area (e.g., 4 yards x 40 yards, 2 yards X 80 yards, etc.), or a field of cavalry caltrops up to 90 square yards in area (e.g., 3 yards x 30 yards, 2 yards x 45 yards, etc.).
Ordinary caltrops make no distinction between friend or foe; all creatures entering a caltrop-sown area suffer the same consequences. The same is true of magical caltrops, with one exception: the casting priest can terminate the spell at any time, causing the caltrops to vanish and leaving the terrain clear.
Unlike normal caltrops, a region sown with magical caltrops cannot be "swept" clear; the magical caltrops remain in place until the spell terminates.

[Tome of Magic]

Sphere: War
(Evocation)
Level: 3

Range: 20 yards/ level
Components: V, S, M
Duration: 1 turn/level
Casting Time: 1 turn
Area of Effect: Special
Saving Throw: None

Material Components:
a golden caltrop