|When activated it hides its wearer from the sight of others, but also increases their metabolic rate by a large amount.|
Wearing equipment with the invisibility ego grants you the Evoke Invisibility ability, allowing you to turn invisible at will. Although it requires a high Evocations skill to use reliably or for a long duration, there are no penalties for failing to evoke it, so even unskilled characters can make some use of it. However, invisibility's benefits come with significant hunger and magic contamination costs, and are less dramatic than one might expect. See the Invisible article for details.
Note that you cannot turn visible once invisible, so beware of magical contamination when using it. This means you should always rest off all your magical contamination between fights. Also note that taking off the invisibility-granting item will not work either; this will just give extra contamination.
The invisibility ego can be found on:
Invisibility is a very useful cloak ego, allowing the player a chance to go invisible at 0 evocations, a coin-flip at 5 evocations, instead of using a potion or learning a level 6 spell to do so. Invisibility grants several bonuses, such as decreased enemy to-hit, the ability for enemies to forget about you, and the ability to get level 2 stabs. This ego proves especially useful in the Swamp, the Shoals and the Spider's Nest, where most enemies cannot see invisible. Just be sure not to enter water, as you will be detected.
Prior to 0.21, removing an invisibility-ego item while invisible did not cost the player extra magical contamination.
Prior to 0.20, this ego would let the player turn visible again once invisible, as a way to reduce contamination.
Prior to 0.16, this was called the Darkness ego.