Death particle

The death particle was a particle that, when activated, was capable of destroying all organic life. The Cyberman Ashad held one in its chest and intended to use to wipe out organic life in the universe and evolve its race in full robots. Eventually, the Thirteenth Doctor indirectly used it to neutralise the Master powered by the Cyberium and its army of CyberMasters before they could leave Gallifrey and conquer the universe. (TV: The Timeless Children)

The Death Particle was a single particle created by the Cyberium, the shared artificial intelligence of the Cybermen, capable of wiping out all organic life on a planet. (TV: The Timeless Children)

Ashad, a half-converted Cyberman who absorbed the Cyberium, contained the Death Particle within the chest unit of his Cyber-suit. As the Cybermen were themselves part organic, they would be susceptible to this particle. Knowing this, they strove to make the final "ascension" to full mechanisation, removing their organic components and becoming completely mechanical, or "robots" as the Spy Master put it. (TV: The Timeless Children)