Invisibility Purge

All invisible creatures who enter an area enchanted with invisiblity purge instantly become visible. Invisibility-related spells do not take effect within the boundaries of the enchanted area, and magical devices such as potions of invisibility do not function. Creatures with the natural ability to become invisible are unable to use this ability within the area of effect. Invisible objects carried into the warded area also become visible. Invisible creatures or persons within the area of effect when invisibility purge is cast remain invisible; however, if such creatures exit the area of effect and later re-enter, they instantly become visible. Such creatures also lose any natural ability to turn invisible as long as they remain within the area of effect.
A creature who consumes a potion of invisibility outside the warded area becomes invisible normally, but becomes visible when he enters the area of effect; if the duration of the potion of invisibility has not yet expired when he exits the area of effect, he becomes invisible again outside the area.
Creatures who are invisible in their natural state or have no visible form (such as invisible stalkers) are not affected by this spell.
The invisibility purge can be cast as a cooperative magic spell. The potency of this spell can be increased if several priests cast it at the same time. The duration of the spell is then equal to one turn per level of the most powerful priest, plus one turn for every contributing priest. Each priest also increases the area of effect by one 10' X 10' square (these areas must be contiguous). Thus, a 9th-Ievel priest and two 5th-IeveI priests could create a 30' x 10' invisibility purge area having a duration of 11 turns.

[Tome of Magic]

Sphere: Wards
(Abjuration)
Level: 3
Solo or Cooperative Spell

Range: 30 yards
Components: V, S, M
Duration: 1 turn/level
Casting Time: 1 turn
Area of Effect: 10-foot square/priest
Saving Throw: None

Material Components:
the priest's holy symbol and a silver mirror no more than three inches in diameter