- They'll never see it coming. 
Invisibility is a Mage spell which renders the Mage unseen in a similar way to Stealth. It is primarily used as an escape mechanism by Mages; as a limited offensive tool; and as an ability to skip content. It has limited uses because the Mage can only see and be seen by players which are invisible themselves or can detect invisibility, and by party members. This does not include stealthed (or Shadowmelded) players, as invisibility differs from stealth.
Three seconds is the length of time it takes a Mage to fade to complete invisibility. The spell cast itself is instant, which means the Mage can cast it on the move, but the Mage will only become invisible 3 seconds after casting the spell. Casting it applies a self-buff that shows the player the countdown. Every second for which Invisibility is counting down, the Mage will lose threat, however, if he chooses to end the fade prematurely, he will lose a minimal amount of threat. Once the timer hits 0, the Mage gains full invisibility, and is removed from combat.
Originally created by clerics. This is the ability to cloud the perceptions of others so that they do not recognize the physical existence of the caster. This was originally employed as a tool to make the confessing of secrets that weighed heavily upon the souls of worshipers easier to speak. The clerics of Northshire Abbey have found it very useful in assisting the armies of King Llane in ridding Azeroth of the Orcs.
Gleaned from sacred tomes rescued from the debris of Northshire Abbey, this spell grants the ability to cloud the perceptions of others so that they cannot perceive the physical existence of the caster’s target. The individual that is rendered Invisible may not perform any tasks such as attacking, harvesting or spellcasting. Should the individual interact with his environment in any fashion more aggressive than simple movement, the Invisibility will be dispelled and their personage shall be revealed.This form of illusion has come back into widespread use since the days of the Second War. It creates a magical field around a person that does not impede the passing of light. Thus, normal people simply see directly through the invisible person. However, any attempts to attack or cast spells while invisible will cause the invisibility to dissipate.
Mage Invisibility does not share a cooldown with Invisibility Potions, so it is possible to attain 38 total seconds of Invisibility if you use an Invisibility Potion during the last second of the Mage effect.
Taking the deep Arcane talent Prismatic Cloak will make Invisibility complete its fade time instantly instead of wasting 3 seconds, which is a very powerful effect and almost mandatory for Arcane PvP.
Coupled with Ice Block AND Invisibility Potion, it can provide a extremely long 48 seconds of complete untouchability time, invaluable for PvP situations and avoiding battles in PvP servers. Thus, the timing must be perfect.
The Invisibility effect will remain intact when the Mage receives beneficial actions (heals, blessings, buffs) from external sources, such as a party member.
If you pull more enemies than you can handle at once, try to Frost Nova them in place and cast blink first, THEN cast invisibility after. Assuming that the mobs don't attack you with a ranged attack you can "reset the fight" and run to a safe location, saving yourself from durability damage and a repair bill.
In raids, while Invisibility can completely dump all your threat, it should be used if at all at the very beginning of the fight after offensive cooldowns (Icy Veins, damage trinkets, etc.) are first used to minimize high-dps time lost. Favor talents like Frost Channeling and Burning Soul to avoid having to stop DPS to wait for the fade time. Under no circumstances waste Molten Fury uptime (boss at 35% health or under) with the fade time, as its damage bonus is too precious. Plan ahead and fade before instead.
In PvP or PvE encounters you can use invisibility to remove yourself from combat allowing you to grab a quick food or drink and replenish your health and mana. Be mindful of the fact you're not DPSing while doing so, and only do this if it is critical and your Mana Gems, Evocation and Mana Potions are on cooldown.