Lynchpin
From Tardis Wiki, the free Doctor Who reference
A lynchpin was a person or thing vital to an enterprise or organisation.
The Monks' 2017 invasion of Earth was stopped when the Twelfth Doctor destroyed the Monks' lynchpin. Previously, Missy claimed to have fought and defeated the Monks on a planet by pushing the Monks' lynchpin into a volcano. (TV: The Lie of the Land)
