After completing Icebrew's Secret Recipe quest he will have you another task. He needs a Dark Elf to test his secret recipe on.
So he gives you the drink and sends you to Longshadow Alley with no directions whatsoever. The dark elf you should give the drink to is Modian K'Jarr and he can be found at -2, 0, -20 . As soon as he drinks the liquid his eyeballs will pop out and he will keel over and die.
Return to Bartender Icrebew with this news for coin and experience.