You can find these Underbelly Elixirs lying around on the floor or in the water of the Underbelly.
There are three possible effects from these which lasts 10mins:
1) Hungry Tuskarr
Turns you into a regular tuskarr who can walk, run, swim, jump, and sit. The great thing is the +50 Fishing bonus, which can stack with baubles, baits and rods. Unfortunately, the buff can only be used in Dalaran.
2) The City of Mages
Turns everyone into a Human Mage with the Tier 2 Mage set. The buff you get is “You had always heard that this was “The City of Mages” but this is ridiculous!” This effect can be seen all over Dalaran.
3) Flying Wasp
You can fly in this form, even if you don’t have any flying skill but will only work in the Underbelly area.