D&Dtools

As protection from arrows, except that if any missile that strikes you has all its damage negated by your damage reduction (10/+1), the missile is turned back upon its target.
The attacker's attack roll is used to determine if the reversed ranged weapon strikes the attacker, but the damage is rerolled.
If the attacker is also protected by a reverse arrows spell, it is possible that the ranged weapon would bounce between them each round until one of the spells is discharged from accumulated damage.
The damage reduction increases with the caster level to 10/+2 at 10th, 10/+3 at 15th, and 10/+5 at 20th.
Once the spell has prevented a total of 10 points of damage per caster level (maximum 100 points), it is discharged.
Focus: A piece of shell from a tortoise, and a ball of tree sap.