Algernon is an Alchemy vendor located in the Apothecarium in the Undercity.
He sells the 
Recipe: Superior Mana Potion to Horde players for 1 20. It's a limited supply recipe, so you have to wait until it's back "in stock" if someone bought it before you.
In-game Picture
Location in Undercity
Coordinates: 51.4, 74.4
TomTom command:
/way Undercity 51.4 74.4 Algernon
