Alarm creates a subtle ward on an area you select. Once the spell effect is in place, it thereafter sounds a mental or audible alarm each time an invisible Creature enters the warded area or touches it. A Creature that speaks the password (determined by you at the time of casting) does not set off the alarm. You decide at the time of casting whether the alarm will be mental or audible in nature.

A mental alarm alerts you (and only you) so long as you remain within 1 mile of the warded area. You note a single mental “ping” that awakens you from normal sleep but does not otherwise disturb Concentration. The spell has a different mental alarm depending on whether the invisible Creature is entering or leaving the area. This spell does not aid you in sensing or pinpointing the location of an invisible Creature within the spell's area. A silence spell has no effect on a mental alarm.

Ethereal or astral creatures do not trigger the alarm.

Invisibility alarm can be made permanent with a Permanency spell by a caster of 9th level or higher at a cost of 2,500 gp.