This spell is the most common way for players to become invisible. Invisibility has many uses, most notably it allows one to avoid attack from most aggressive monsters and even walk right past many blockers.

The in game help reads:

No saving throw.
Casting time: 1 round.
Type of spell: neutral spell.
Affecting stats: wis.
Cast type: transformation.
Spell point cost: 63.
One of the most useful spells from the illusion  schools, is the spell of
invisibility. This spell will  make the target and all his possesions
invisible to the naked eye. There is no better way to go places you normally
shouldn't be caught in. Of course there are also magical ways to detect
invisible beings, this spell does not offer total impunity.