Light of Elune

The Light of Elune was a quest reward from Mage Summoner.

Operation
This potion creates an effect similar to a paladin's Divine Shield, making the user essentially invulnerable for 10 seconds. It clears negative debuffs upon use. It does not weaken or prevent any combat actions such as spellcasting or damage dealing. However, certain environment mechanics such as flag-carrying or chest-looting may be restricted to prevent exploitation, as they are with other immunity effects.

The item is consumed when used.

Legacy
Light of Elune is infamous as one of the rewards in the classic game that is potentially exploitable by players, but was included early in the development process before designers were as experienced in foreseeing consequences of new abilities and effects. While the effect lasts only briefly, the potion's ability to provide total immunity with almost no restrictions on actions makes it an incredible asset if used at a key moment. A raid of players equipped with Lights of Elune, for instance, could bypass a critical survival moment in a fight and dramatically increase their chances of success beyond what was intended. It could easily tip the scales in a Player vs. Player fight.

The potion as originally implemented did not become soulbound upon receiving it, allowing it to be bought and sold. Some players would even create alts specifically to level them up to 25, obtain the Light of Elune, and send the potion to a main character. The potions commanded great value on the auction house. Eventually, the potion was changed to bind on pickup, limiting its impact to one use in the entire lifespan of a character. As of World of Warcraft: Cataclysm, neither it or any similar item can be obtained anymore.

Given the low level of the quest granting it, the developers likely imagined the Light of Elune would be used in a more casual way during the leveling experience. Other items with seemingly small but surprisingly desirable effects even at higher levels include the Bag of Marbles, Glowing Wax Stick, Slumber Sand, Really Sticky Glue, and Large Rope Net. All were eventually changed to bind on pickup, and most were ultimately removed.

In addition to its place in development history, the Light of Elune is regarded by many as being emblematic of different players' styles. As the ultimate high-impact, single-use item, its use is representative of any item or event with a unique effect but a limited supply. Some players use the potion almost immediately, possibly wasting its potential. Others may save it for a more well-considered moment, and hopefully get a nice small victory from it, as Blizzard likely intended. Others may put a great deal of thought into how best to exploit it in unexpected ways, although with the one-time-only limitation the payoff for this is unlikely to be too great. Finally, a certain type of player will try to save it for "the perfect moment". For many of these, that moment will never come and it will remain in their bags (or bank) forever. The pressure of accidentally wasting it after so much time may simply be too much. But a few have reported using the Light of Elune, years and years later, in a key fight when their group and the enemy are both a breath away from defeat, for one last push for victory, finally rewarding them for all their patience with the accolades of friends and teammates at how well the potion was ultimately used.

Trivia
The Light of Elune has been compared to Pokemon's Master Ball.