The Elixir is a small mission made by many hands, a sort of community project centered around the shiny new (at the time) module set created by Springheel. Along with "A New Job" these missions worked as kind of a showcase for those modules, and this mission was "built on YouTube" by Bikerdude, as a tutorial for new (and old) mission makers, which was a nice touch.
The mission itself is straightforward. A local merchant who is looking to climb the social ladder has gathered a bit of a cult following around his parties, which include the titular elixir, which we are to find the recipe for. The man in question lives in a cramped town house in a dark back alley, and he is wealthy enough to hire a couple of guards and manservants. The gameplay is good as navigating the house is pretty challenging, and we have to figure out where the recipe of the elixir is stored while staying undetected. The street section is relatively easy to navigate, but it certainly has a thick atmosphere to it, while also providing a few hidden areas to explore. I would say that this mission transcends it's intention of being a showcase/tutorial and can stand on it's own merits as a fun but relatively small mission, which is well worth a playthrough even for those who have no interest in learning how to make missions for TDM!