Ritskaw

Ritskaw was a Sontaran Commander. He was an "old friend" of Psychic Surveyor Kragar.

Biography
After finishing his investigation of the Flux, Kragar called Ritskaw on his ship, informing him of a greater cataclysm than had ever been known, and deciding to use the opportunity to strike other species while they were at their weakest; Ritskaw was excited by the prospect. As Earth was bearing the brunt of the Flux cloud, Ritskaw ordered the attack. (TV: The Halloween Apocalypse)

The Sontarans managed to invade Earth, through a small gap, in the fractions of a second before the Lupari fleet had fully shielded the Earth. Ritskaw commanded the invasion of Earth in 2021, from their base in Liverpool, where they were building time ships to travel back in time, and conquer specific moments in Earth's history, turning Earth into an outpost for the Sontaran Empire from the dawn of time, and erasing the history of humanity as a result. Dan Lewis snuck aboard the command ship, where he made contact with the Thirteenth Doctor and Mary Seacole in 1855, when Ritskaw intercepted their transmission and captured Dan. He was about to execute Dan for spying, before Karvanista appeared and shot Ritskaw and his soldiers. Ritskaw was last heard on the Sontarans' comms, before the command ship, flown by a fleeing Dan and Karvanista, flew straight into the other time ships, causing a temporal explosion that wiped out any existence of the Sontarans occupying the Earth at that time. (TV: War of the Sontarans)

Behind the scenes
In the credits for The Halloween Apocalypse, this character's name was spelled as "Ritskaw". In the following episode, War of the Sontarans, in which the character features more prominently, he is miscredited in the credits and on the BBC iPlayer subtitles at the 52 minute mark as "Riskaw", despite being named onscreen in the episode itself as "Ritskaw".