The Lucian serum is the serum given to Luke Cahill. Taking the serum will grant the consumer all Lucian Branch powers and abilities, including strategic plotting, spying abilities, and hunger for power. Benjamin Franklin is the only known Lucian to successfully make the serum. Luke Cahill, Isabel Kabra, and possibly Vikram Kabra are the only known people to have taken it. Vikram Kabra, however, was born with the serum in his DNA, while Isabel wasn't. She was born a Vesper.
The Lucian serum is composed of one portion of each of the following ingredients:
Dan and Amy find Benjamin Franklin's bottle of Lucian Serum in a secret compartment of the Catacombs of Paris, before finding out that Jonah was spying on them. Eventually, Ian Kabra ties up Dan without Amy knowing, and when Amy discovers so, he tells her that he poisoned Dan and will give her the antidote in exchange for the serum. Since Dan is unable to talk, she obliges, and Ian takes the serum. When she unties Dan, he tells her that Ian in fact didn't poison him.
The serum itself doesn't appear in this book, but Ian finds the empty bottle in the secret wing of the Kabra Mansion and realizes that Isabel already drank it. Further on in the book, Isabel admits she drank it and proves she has "stunning" Lucian powers, as she demostrated when kidnapping a friend or family member of each Cahill branch.