The death particle was a weapon created by the Cyberium and Ashad, as part of an attempt by Ashad at "ascending" the last surviving Cyber-Warriors into a purely robotic species. (TV: The Timeless Children)
The death particle was created by the Cyberium "through" Ashad. It was a tiny, glowing red particle that pulsed like a heartbeat, and was contained within the chest unit of Ashad's Cyber-body. Ashad strongly implied that, upon detonation, the death particle would destroy all organic life in the universe, but after the Spy Master used his Tissue Compression Eliminator to shrink Ashad and the death particle, the weapon's range was limited to a single planet. The death particle could be detonated by a powerful explosive at point-blank range, but could not be detonated by the Master's Tissue Compression Eliminator. (TV: The Timeless Children)
The death particle was created in the immediate aftermath of the Cyber-Wars of the far future by the Cyberium, which had been absorbed by Ashad. Having reactivated an army of tens of thousands of Cyber-Warriors contained within a dormant Cybercarrier, Ashad intended to detonate the death particle to destroy all organic life in the universe; to ensure the Cyber-Warriors' survival, Ashad erased their organic components, transforming the Cyber-Warriors into a purely robotic species, and intended to do the same to himself prior to the death particle's detonation.
After arriving on Gallifrey by invitation of the Spy Master, Ashad unveiled his plan to the Time Lord, but the Master dismissed Ashad, claiming that the existing dominance of robots across the universe meant that Ashad's "ascension" of the Cybermen wouldn't make them the true dominant species. After offering an alliance, the Master and Ashad travelled to the Cybercarrier's conversion chambers, where the Master shrank and killed Ashad with his Tissue Compression Eliminator; the Master expressed disappointment at the death particle not detonating, before absorbing the Cyberium and leaving the shrunken death particle within Ashad's miniature corpse to be found by the Thirteenth Doctor.
Upon awakening in the Citadel, the Doctor was informed of the death particle by Ravio, Yedlarmi and Ethan. Finding Ashad's corpse within the Cybercarrier, the Doctor returned with it to the Citadel, and intended to detonate one of Ko Sharmus' explosives to detonate the miniaturised death particle, thereby killing the Master, the CyberMasters, created by the Master and the Cyberium, and herself. However, Ko Sharmus then arrived and convinced the Doctor to allow him to detonate the death particle, allowing the Doctor to escape Gallifrey in a spare TARDIS. Shortly afterwards, Ko Sharmus detonated the death particle just before he died after being shot by the CyberMasters, although the Master and the CyberMasters attempted to escape. (TV: The Timeless Children)