Mass Invisibility

This is a more extensive adaptation of the invisibility spell for battlefield use. It can hide creatures in a 60 yd. X 60 yd. square area: up to 300 to 400 man-sized creatures, 30 to 40 giants, or six to eight large dragons. The effect is mobile with the unit and is broken when the unit attacks. Individuals leaving the unit become visible. The wizard can end this spell upon command.

[Player's Handbook]

School: Illusion/Phantasm
Level: 7
Rarity: Common

Range: 10 yards/ level
Components: V, S, M
Duration: Special
Casting Time: 7
Area of Effect: Special
Saving Throw: None

Material Components: an eyelash and a bit of gum arabic, the former encased in the latter (consumed)