Hélène Jégado, Brittany's Deadliest Servant Inspector Marcel Corentin's methodical investigation of Hélène Jégado reveals one of history's most prolific serial poisoners-a seemingly pious domestic servant who murdered at least 26 people across Brittany between 1833 and 1851. Operating with impunity for eighteen years, Jégado exploited the perfect combination of circumstances: her trusted position in households, the resemblance of arsenic poisoning to common illnesses, fragmented record-keeping across rural parishes, and the protection her excessive religious devotion provided against suspicion. The breakthrough comes through scientific advancement-the Marsh test makes arsenic detectable for the first time-and Corentin's innovative cross-jurisdictional investigation techniques. Though legal limitations allow Jégado to be tried only for three recent murders in Rennes, Corentin meticulously documents her entire criminal career. Executed in 1852, Jégado never confesses to her earlier crimes. Her story gradually transforms in Breton culture from historical criminal to folkloric figure known as "Fleur de Tonnerre." Corentin's comprehensive investigation becomes a blueprint for modern detection methods, demonstrating how pattern analysis, forensic science, and psychological understanding can expose predators who hide behind masks of virtue while dealing death with invisible weapons.