The Madrigal Serum Clues are needed to make the Master Serum safe and stable, as the serum itself without the Madrigal Branch component will kill the drinker eventually, as shown in The 39 Clues Book 11: Vespers Rising. Gideon Cahill drank the serum without the Madrigal Clues and would have killed him if Damien Vesper didn't beat him to it. These Clues were kept with Gideon and Olivia Cahill and were passed down to Madeleine.

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