Invisibility

This spell causes the creature touched to vanish from sight and be undetectable by normal vision or even infravision. Of course, the invisible creature is not magically silenced, and certain other conditions can render the creature detectable. Even allies cannot see the invisible creature or his gear, unless these allies can normally see invisible things or employ magic to do so. Items dropped or put down by the invisible creature become visible, items picked up disappear if tucked into the clothing or pouches worn by the creature. Note, however, that light never becomes invisible, although a source of light can become so (thus the effect is that of a light with no visible source).

The spell remains in effect until it is magically broken or dispelled, until the wizard or recipient cancels it, until the recipient attacks any creature, or until 24 hours have passed. Thus the invisible being can open doors, talk, eat, climb stairs, etc., but if he attacks, he immediately becomes visible, although the invisibility enables him to attack first. Note that the clerical Bless, Chant, and Prayer spells are not attacks for this purpose. All highly Intelligent (Int 13 or more) creatures with 10 or more Hit Dice or levels of experience have a chance to detect invisible objects (they roll saving throws vs. spell; success means they noticed the invisible object).

Commentary

Wizards should keep in mind that invisibility is an illusion, a condition of artificial reality-- it's not an enchantment that causes the caster (or creature of the caster's choice) to literally disappear. As such, invisibility is subject to all of the advantages and limitations of illusionary magic. Following are the most pertinent points:

• The invisible character can always see himself; in effect, he automatically disbelieves the illusion.
• The caster cannot tell with certainty which observers have successfully disbelieved his illusion of invisibility; he can only be sure by observing their reactions.
• All objects on the invisible subject's person are also invisible. However, the DM might wish to impose limitations, since it might not make sense for a 30-foot pole carried by an invisible character to also be entirely invisible. The DM might assume, for instance, that the illusion extends to a 1-foot or 2-foot radius around the affected subject; hence, all objects in contact with the subject that are within this area are also invisible. If such a limitation is imposed, then an object extending beyond this range (such as the 30-foot pole) is entirely visible; partially invisible objects aren't allowed.
• Not every creature with fewer than 10 Hit Dice is excluded from noticing an invisible character. Creatures with excellent hearing or those with a sensitive sense of smell have a chance of detecting an invisible character. The DM should take such heightened senses into account when determining which creatures are allowed saving throws.
• Opponents do not automatically get a chance to detect the presence of an invisible creature. They must first have reason to believe an invisible creature is among them. For instance, a creature might have abruptly disappeared, or the opponents might hear a mysterious sound or notice footprints in soft ground. If one opponent successfully sees an invisible creature, he can alert his companions to the creature's presence, at which time they are allowed saving throws to detect the creatures if they haven't made such throws already.

[Player's Handbook]
[Complete Wizard's Handbook]

School: Illusion/Phantasm
Level: 2
Rarity: Common

Range: Touch
Components: V, S, M
Duration: Special
Casting Time: 2
Area of Effect: Creature touched
Saving Throw: None

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