The Master Serum is the serum that is created when all four Branch Serums and the Madrigal Branch Clues are combined in the right proportions. If the Madrigal Clues are not added, a lethal and deadly Master Serum is created instead.
One portion = one ounce
Start with one portion of water.
The Master Serum does in fact do what it was created for—to cure and prevent the plague of the late 1400s and early 1500s. Even though the Black Death is gone, the Serum remains invaluable because curing the plague isn't its only use. It makes the person who drinks it more intelligent, powerful, strong, athletic, artistic and cunning; essentially making the imbiber the ultimate sentient being. It combines and intensifies the traits of all four branches and makes the person taking it almost superhuman. It makes people effectively perfect, which was one of the main pursuits of alchemy.
The serum's creator, Gideon Cahill, divided his creation into four seperate components and then handed one for each of his children based on their individual qualities that correspond to those their portion of the serum bestow. Individually, each portion also seemed to have enabled the drinker the capability to pass on their distinguished traits over to their descendents
The Janus part of the serum would make the drinker the greatest author, musician, singer, song-writer, play-writer, poet, artist, sculptor, comedian, actor, dancer, chef, model, fashion designer, magician, martial arts genius and the person with the greatest creativity and greatest talent to do anything that is even slightly artistic. The Lucian part of the serum would make the drinker the most cunning, sneaky, strategic, leaderful and ruthless person on the planet. The Ekaterina part of the serum would make the drinker the most fantastic inventor, scientist, and make them the most intelligent human in all fields of knowledge. The Tomas part of the serum would make the drinker the greatest adventurer, explorer, and athlete. They would also become the strongest, bravest person on Earth and become unbeatable at all sports.
In short, the full Master Serum would effectively make the imbiber the very best at virtually every single thing human beings are capable of being good at.
Gideon Cahill was the only person to have taken the master serum. The serum almost killed him after using it. This was due to the absence of certain ingredients. Later on Dan Cahill started collecting ingredients for the serum, just in case Vesper One breaks the deal (which is killing the hostages). At the beginning of Book 6 Dan finishes assembling the serum and drinks it. However, Amy had replaced it with a fake. However at the end of Day of Doom, Isabel Kabra steals Dan's emergency serum and drinks it. After she found out that Vesper One killed her daughter, she uses her extreme power gained by the serum and destroys the Machina Fini Mundi and kills Vesper One in the process.
J. Rutherford Pierce, an american media conglomerate in the United States; hired Sammy Mourad, a postgraduate student at the Columbia University to give him a more stabilized version of the master serum by acting as Fiske Cahill who claims he wants to put it in the Madrigal archives. He later kidnaps Sammy and takes him to Wellington labs, Delaware, where he is going to mass manufacture the Master Serum.
It is proven in Unstoppable that the original Master Serum has deadly symptoms. It causes Buccoglossal Syndrome (uncontrollable spasming), Xanthopsia (yellow vision), and hallucination. Also, after one week, the imbiber of the Serum dies.