Invisibility

The invisibility 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: This version of invisibility originally appeared in the 2nd Edition Player's Handbook for Advanced Dungeons & Dragons from TSR. Its use here is for the purposes of providing context for the campaign only.