The Master Serum is the serum that is created when all four branch serums and the Madrigal Clues are combined in the right porportions. If the Madrigal Clues are not added, a lethal and deadly Master Serum is created instead.
For Gideon Cahill's serum: 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, artistic, and cunning. 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 Janus Serum would make the drinker an amazing author, musician, singer, song-writer, play-writer, poet, artist, sculptor, comedian, actor, dancer, chef, magician, martial arts genius, a person with great creativity and great talent to do anything that is even slightly artistic.
The Lucian Serum would make the drinker cunning, sneaky, strategic, leaderful, ruthless and good with plots.
The Ekaterina serum would make the drinker a fantastic inventor, scientist and make them extremely intelligent in all fields of knowledge (including being fluent in every language, or at least easy to learn them)
The Tomas Serum wouuld make the drinker a great adventurer, explorer and athlete. They would also become very strong, brave and excellent at all sports.
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. At the end of Day of Doom, Isabel Kabra steals Dan's emergency just-in-case-Amy-switches-it copy of the serum and drinks it. She uses her extreme power to destroy the Machina Fini Mundi and Vesper One. (Isabel turns good after she found out Vesper One killed her daughter.) Isabel is the only person ever to have drank the Master Serum with Madrigal Clues.