Razing of Gallifrey

The planet Gallifrey was razed to the ground by. (TV: Spyfall) The Thirteenth Doctor believed it to be the second destruction of Gallifrey, following the so-called fall of Gallifrey at the end of the Last Great Time War. (TV: Fugitive of the Judoon)

A day to come
Having been made aware of 's razing of Gallifrey, the Thirteenth Doctor reasoned that both the Gallifreyan Gat and the Fugitive Doctor were from her past as Gallifrey had been destroyed "by a lunatic" by her time, which she disclosed to the two Gallifreyans to the point of showing Gat her mind's image of the ruined Gallifrey, which was dismissed as "trickery". (TV: Fugitive of the Judoon)

Discussing the Gallifrey Falls No More painting, which concerned the first fall of Gallifrey, with Doctor Henry Black, the Curator told him it would be better described as "Gallifrey Falls No More (Until the Next Time)"; (PROSE: Dr Black) as a future incarnation of the Doctor, (COMIC: The Then and the Now) the Curator knew that strife would return to Gallifrey after the Time War. (PROSE: Dr Black, TV: The Timeless Children)

Background
Despite living as Missy, a life where the Master had hoped to do good in the universe, (TV: The Doctor Falls) the Spy Master returned to his dark ways. (TV: The Timeless Children) Though the Twelfth Doctor observed that Gallifrey, having been relocated to a pocket universe at the end of the Last Great Time War, had returned to the Doctor's universe at the end of the universe, (TV: Hell Bent) the Master claimed to the Thirteenth Doctor that Gallifrey was still hiding in its "little bubble universe" when he returned to it. (TV: Spyfall)

Whilst there, he hacked into the Matrix where he found that Gallifrey was built from the Timeless Child, a foundling from an unknown realm who was adopted by the early Gallifreyan explorer Tecteun, who derived the ability of regeneration from the child, who would eventually become the Doctor. Angered that the Founders of Gallifrey had lied, the Master sought to "make them pay" and proceeded to raze Gallifrey, apparently killing all other Gallifreyans. Anticipating that they could be useful, he made sure to preserve the Time Lords' corpses, (TV: The Timeless Children) and helped himself to advanced Gallifreyan technology. (TV: The Power of the Doctor)

Aftermath
During the Kasaavin invasion of the universe, the Master made the Thirteenth Doctor aware that "someone" had destroyed Gallifrey. After she visited Gallifrey and saw the planet in ruins, the Doctor found a holographic message from the Master in the TARDIS in which he admitted responsibility. (TV: Spyfall)

Eventually, the Doctor, whilst embroiled in the restoration of the Cyber-Empire, came across the Planet of the Boundary leading to Gallifrey. Emerging from the Boundary, (TV: Ascension of the Cybermen) the Master forced the Doctor to come with him to Gallifrey, where he revealed to her that she was the Timeless Child. When a Cybercarrier arrived on Gallifrey, the Master convinced the Cyberium, the artificial intelligence of the Cybermen, to take him as host, enabling him to use the resources of the Cybermen to convert the preserved Time Lord corpses into CyberMasters, modified Cyber-Warriors capable of regeneration. Ultimately, the Doctor escaped whilst Ko Sharmus detonated the death particle, wiping out all life on Gallifrey. (TV: The Timeless Children)

However, the Master survived and continued to lead the Cybermen. Forming an alliance with the Daleks, the Master conceived an ill-fated plot to eliminate the Doctor by means of a forced regeneration, the technology for which he had acquired when he ransacked Gallifrey. The scheme ended with the defeat of the CyberMasters and the Master himself being gravely wounded. (TV: The Power of the Doctor)

Behind the scenes

 * Simon Bucher-Jones suggested that the Gallifrey which was razed by the Master was one of the seven surviving duplicate Homeworlds.