Master Serum

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.

Serum recipe

For Gideon Cahill's Serum:

One portion = one ounce

Start with one portion of water.

Add 1/8 portion of each of the ingredients of the Lucian Branch. (So it could make you have lust for power, which is bad, the most)

Add 1/16 portion of each of the ingredients of the Janus Branch.

Add 1/24 portion of each of the ingredients of the Tomas Branch.

Add 1/32 portion of each of the ingredients of the Ekaterina Branch.

Add 1/20 portion of each of the ingredients of the Madrigal Branch. 

Powers

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 separate 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 descendants.

The serum is frequently described throughout the series as being capable of making the imbiber the most powerful person in the world. The serum gives extreme talent to the imbiber in almost all areas of life, due to the combined effects of the four branch serums. The Janus part of the serum would make the drinker incredibly talented things such as writing, music, poetry, art, comedy, acting, singing, dancing, cooking, performing and other variously artistic skills. The Lucian part of the serum would make them extremely cunning, sneaky, strategic, a leader, and ruthless. The Ekaterina part would make them a fantastic inventor, scientist and give them advanced mental abilities and thus could easily become highly proficient in all fields of knowledge. Finally, the Tomas part would make them a great adventurer, explorer, athlete and give extreme strength and bravery. 

The serum likely also improves the imbiber's health and appearance, since when Damien Vesper first sees Gideon after Gideon takes the serum, he notices that Gideon appears healthier and younger, that he stands straighter and that his hair appears darker.

Based on Amy and the Piercers, the serum shows to maximize the body's physical and mental capabilities, not making them superhuman but as powerful in all ways as a human could be, making them peak human. This is broken down into the four branch serums and tied together by the fifth, enhancing individually strategy and cunning, intelligence and inventiveness, strength and speed, and talent and creativity. Despite this, it is shown that when mixed together, the individual effects are even stronger than each single serum. Although together, the serum is also shown to increase senses and reflexes, enhancing one's sight and hearing along with their reaction time, which aren't normally shown to be in the individual serum. It also gives one an inner glow along with their more youthful appearance, although this isn't shown as significantly when Amy and the Piercers take it than with the Pierces themselves, as Debi Ann looks much older and weaker than her family because they had been mixing the serum into their power shakes.

Uses

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. The serum colour can be questioned, as at the end of Trust No One it is described as "a cloudy amber liquid", but in Day of Doom, it is described as a red and green liquid. At the end of Day of Doom, Isabel Kabra steals Dan's emergency serum and drinks it. She turns red like the serum, because Dan put in red M&M's for taste. 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 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 Trilon labs, Delaware, where he is going to mass manufacture the Master Serum. Sammy sends a sample of the original version of the serum to Amy Cahill in Breakaway. In Countdown, Amy drinks the serum to save Dan from Pierce’s goons, but in return starts dying.

In Mission Hurricane, Magnus Hansen, the Tomas leader is caught by Nellie Gomez and Sammy Mourad, taking the Tomas clues from the Tomas base on Mount Fuji to give to Nathaniel Hartford. In Mission Atomic, it is revealed that Nathaniel Hartford has all the clues and only need the serum formula, So he kidnaps Dan, then creates a deadly poison to kill all Cahills in the form of bees able to kill all Cahills they sting.

Symptoms

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 three weeks, the imbiber of the Serum dies.