It may look like Princess Diana’s only daring moment happened after she confronted Prince Charles and Camilla Parker-Bowles about their secret affair, but that is not true. There was a time when the Princess of Wales threatened to have the mistress of her then husband killed via anonymous late-night phone calls.

A new royal biography about the Duchess of Cornwall titled “The Duchess: The Untold Story” reveals that while suffering in silence because of Prince Charles’ betrayal, Princess Diana made bold moves to threaten the Duchess of Cornwall. In the middle of the night, she would call Camilla to tell her that someone was going to kill her.

“Without saying who was calling, she’d typically say: ‘I’ve sent someone to kill you. They’re outside in the garden. Look out of the window, can you see them?” royal author Penny Junor revealed.

Junor also revealed that Diana would leave the same threats to other people’s answering machines and pagers. In one instance, she left a note to her private secretary, Patrick Jephson. “We know where you are, and so does your wife. I know you’re being disloyal to me,” she wrote.

According to Junor, the Duchess of Cornwall liked Princess Diana at first. In fact, the two managed to form a genuine friendship despite Camilla’s past relationship with Prince Charles. Months before Princess Diana’s royal wedding to Prince Charles, however, Princess Diana started to feel resentment toward Camilla and became jealous of their past relationship. That was when Camilla decided to distance herself from the royal couple.

In the course of her marriage to Prince Charles, several incidents further encouraged Princess Diana’s jealousy. One time, the Princess of Wales found a photo of Camilla fall out of her husband’s diary. She also noticed the gold cufflinks with interwoven Cs often worn by Prince Charles during formal dinners. All along, she knew that Prince Charles was still in love with his ex-girlfriend.

Despite Princess Diana’s efforts to stop their affair, there was simply no stopping Prince Charles and Camilla. After Princess Diana’s divorce from the Prince of Wales, Prince Charles continued his affair with Camilla until they got married in 2005.

Princess Diana A wax figure of Princess Diana is seen during the official Madame Tussauds' wax effigy of HRH Prince William is unveiled at London's Marylebone Road tourist attraction on August 3, 2005, in London, England. MJ Kim/Getty Images