Invisibility, or incorporeality, is a mechanic that renders the owner unseen. Unlike stealth, it is of magical origin. There is no chance to detect an invisible target without a detection effect. However, detection effects reveal invisible units at any range. Even when detected, invisible units are transparent and do not display a name tag.
Just like Stealth, invisibility comes undone if you take most actions other than movement. This does not necessarily apply to mobs, most remain invisible when attacked. Unlike Stealth your movement speed is unaffected while invisible. While invisible, the player can only see and be seen by players which are invisible themselves or can detect invisibility, and by party members.
There are only a few items in the game that provide invisibility. They are: (requires level 23 to use; lasts 15 seconds with a 10-minute cooldown), (requires level 37 to use; lasts 18 seconds with a 10-minute cooldown), and (requires 200 Engineering to use; lasts 10 seconds with a 1-hour cooldown). The only spells which allow invisibility are the Mage's Invisibility and the Warlock's Succubus' Lesser Invisibility. All currently available invisibility effects are limited to their caster.
There are several invisible mobs in the world. There are the ghosts in the Ruins of Lordaeron in Tirisfal Glades, the ghosts in Raven Hill in Duskwood, the shades of Naxxramas the infamous invisible patrols in Dire Maul West.
Other than the Dire Maul and Eastern Plagueland ones, however, most invisible mobs of the world tend to be neutral, and won't attack unless provoked, which means that you wouldn't know they were there unless you had invisibility detection or happen to hit them with an AoE (in which case they will engage you, but remain invisible).
There are items and spells in the game that provide the ability to see invisible beings. Prior to WoW 2.0, there were three separate grades of invisibility, but currently there is only one. Likewise, Warlocks previously had three Detect Invisibility spells, but currently there is only the one: Detect Invisibility, acquired at level 26. This buff can be cast on any ally, not just the Warlock, and lasts 10 minutes.
Mobs with invisibility detection
Each of the major cities has patrolling guards with Stealth and invisibility detection abilities, and some mobs in instances (some were added to Stratholme in patch 1.6, for example) can also detect some grade of invisibility.
Tables of invisibility and detectionThis section concerns content exclusive to The Burning Crusade.
|100||5 min||Succubus' Lesser Invisibility|
|100||10 min||Pre-BC Detect Lesser Invisibility|
|200||10 min||Pre-BC Detect Invisibility|
|300||10 min||Pre-BC Detect Greater Invisibility|
|1000||10 min||Detect Invisibility|