The death particle was a single particle created by the Cyberium that, when activated, was capable of destroying all organic life on at least one planet.
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 surmised. (TV: The Timeless Children)
The Thirteenth Doctor intended to utilise it to neutralise the Spy Master and his CyberMasters, however Ko Sharmus utilised the death particle, allowing the Doctor to escape. (TV: The Timeless Children)