Elixir is the nineteenth optional quest. It is obtained by talking to Fire the Pandaren Merchant in Kalm. It is completed by simply finding an Elixir and bringing it to Fire, who then rewards the players with 2000 Gold and 500 EXP. Three minutes later, Fire starts selling Elixirs in his stock, which is also marked in an entry in the Kalm News.
The quest also affects a particular entry in the Kalm News. After two and a half days have passed on the game's clock Kesha's Special Brew is voted best drink in Kalm. If this quest is complete by then the Elixir is voted second best, if not it's the Nectar instead.
Completing the quest also unlocks the Fire's Stock miniquest. If you find potions that are not sold in his shop already, try selling 1 to him and he may ask for more to research! Continue to do this til he tells you that he'll study this potion. After a few minutes, his stock will update and you can now buy them!