This buff is gained when the 6 people with the unstable potion buff is hit by one of the mobs that casts Breath of Atrebe.
In the southern room behind the Snowblaze, you will find 6 targetable boxes mixed in with the zone’s decorative static boxes. The player who breaks a box open will receive a 5 minute buff called Unstable Potion. That person with the Unstable Potion buff will want to position themselves to intentionally get hit by a Kar`Zok's Breath of Atrebe spell, which will then trigger the Unstable Infusion buff. Acquiring the Unstable Infusion buff is the necessary key for unlocking the Essence of the Dragon II (Raid) achievement, granting immunity to the zone’s Aura of Kor-Sha. Note that the box places the buff on the box breaker, so it is very important that you do not use pets to crack a box open! Also note that the achievement is not awarded until event completion, so you cannot farm boxes. Finally, do not die after receiving Unstable Potion but before receiving Unstable Infusion, or you will have wasted the box. Dying after Unstable Infusion does not matter, the achievement is still awarded on completion of the event.
Something inside the box breaks as it does, spraying the one breaking the box with some sort of liquid. (box break, everybody can see this)
You are covered in an unstable magical liquid. (Unstable Potion spell emote for box breaker)
The potion and magic from the sarnak’s breath meld and infuse. (Unstable Infusion spell emote)
The unstable liquid melds with the magic of the Kar`Zok’s breath. You feel the substance infuse your skin. It tingles. (Unstable Infusion success)