The Ekaterina Serum is the serum of the Ekaterina branch. Taking this serum will grant the user immense intelligence and inventiveness. It is composed of the following:
Gideon Cahill completed the master serum by 1507. He gave the Ekaterina Serum to Katherine Cahill, and she hid the ingredients as Clues. Katherine also was forced to take one of Thomas's, her brother's, clues after he married a Portuguese woman & held them in Portugal for 3 years. This led to the Ekaterinas & Tomas to be enemies. However, the Ekaterina retained their knowledge, not losing track of any Clues. Robert Cahill Henderson was considered one of the closest to remaking this serum, as though he had only the first 38 Clues but ironically, he couldn't find the last one until his laboratory was destroyed. It is unknown if anyone, except Katherine, has drank the serum, because even though Henderson had the 38 Clues, the one he was missing was Ekaterina. It also contains the least amount of clues(Only 7 clues to complete the Ekaterina serum), but also after the terrible accident he still tried.
This is the easiest serum to make on the 39 Clues website, because four of the ingredients can be found in missions, two in books, and only one in a card combo, and even this can be obtained by using a replicator card that you will regain for finding the Clue, anyway.