A Sontaran flagship was a massive spacecraft that served as the Sontarans' primary military vessel.
The ship was spherical with eight curved claws, four above and four below. For battle, the top and bottom claws could interlock to reveal a cannon that could obliterate a planet. Around the middle of the sphere were docking ports for dozens of Sontaran scout ships. According to the Tenth Doctor, Earth's entire nuclear arsenal wouldn't even scratch the ship's defences.
One flagship, commanded by General Staal, led the Sontaran Empire's Tenth Battle Fleet. The flagship arrived in orbit of Earth to spearhead an invasion that would turn the planet into a cloning world. It was linked to the surface via teleport pods between the ship, the ATMOS factory and the Rattigan Academy. The ship was destroyed by Luke Rattigan, when he activated his atmospheric converter on the bridge. (TV: The Sontaran Stratagem, The Poison Sky) One Sontaran, Commander Kaagh, in flight near the flagship, survived and landed his scout ship on Earth. (TV: The Last Sontaran)
The Sontarans joined The Alliance formed to imprison the Eleventh Doctor in the Pandorica to save the Universe. At least one Sontaran flagship was in the Alliance fleet which arrived at Stonehenge in 102 AD. (TV: The Pandorica Opens)