Invisibility

Spell effects
Invisibility causes the caster to become invisible to all enemies and lasts indefinitely until removed. While invisible, enemies will treat the caster as though he is not present. Invisibility is removed by the following:
 * Touching an enemy (being in melee range of an enemy).
 * Taking damage of any kind, even 0 damage, includes falling damage and damage diverted to an active personal shield.
 * Casting a spell, Magick or staff ability.
 * A melee attack (both imbued and non-imbued).
 * Nullify Magick.

Invisibility is not removed by the following:
 * Queuing elements into the spellbar.
 * Moving and walking.
 * Changing areas and scenes.

Since Invisibility is removed by taking damage of any kind, it does not work during Rain/Thunder Storm and Blizzard because those weather effects apply a 0 damage water/cold attack respectively to all units every 0.25 seconds over their duration unless the caster is in a shield bubble or is immune to water/cold. Projectile spells, arrows and bombs thrown by enemies before the caster is invisible will still hit the caster after he is invisible.

Invisible units still take up space and other units cannot walk through them. Allied units that are invisible are easily identified by having an outline around their character model which is transparent, similar to the effect seen on players when they walk behind an object that blocks line of sight from the camera.

There are currently no enemies that can "see" invisible units, however attacks like Fafnir's fire beam target the player regardless of a creatures sight.

Achievements
The following achievement requires the use of Invisibility:

Pros

 * Allows the player to skip large areas of enemies by avoiding them.
 * Lasts an indefinite amount of time.
 * Excellent panic button.

Cons

 * Invisibility can be removed by casting Nullify.
 * Cannot be used while Rain/Thunder Storm or Blizzard are active unless protected from their effects appropriately.
 * Useless in PvP because players are considered allies.
 * Cannot be used with certain staffs, such as the Staff of Life.

Trivia

 * Invisibility breaks a lot of triggers and cutscenes in the game because many of them do not trigger while the player is invisible. This can be abused for speed runs.
 * Before patch 1.5.6.0, Invisibility was not removed by the first non-imbued melee attack made by the caster, but only by the second (an imbued melee attack always removes Invisibility). This had major implications for the M60 weapon which fires for an unlimited duration and is considered a "melee attack". A player was then able to cast Invisibility, then use his M60 while invisible indefinitely by holding down Shift+Left Click since its the first "melee attack".