A being who is invisible will not show up when you <look> in a room, nor will they show any messages when entering or leaving a room. Any followers (whether friendly or hostile) will not be able to follow an invisible being when they move. Many skills (though not all) cannot target an invisible being.

There are several ways of becoming invisible, including race skills like stealth and shadowwalk, and guild skills like hide and others. There are also items that enable invisibility...