Staal

General Staal "the Undefeated" of the Tenth Sontaran Battle Fleet was a Sontaran general during the Sontaran invasion of Earth in 2009. His nickname of "Undefeated" was remarked by the Tenth Doctor as not a very good one as it could not remain if he was defeated and jokingly suggested "Staal the not-quite-so-undefeated-anymore-but-never-mind" as a replacement.

Biography
In 2009, Staal forged an alliance with Luke Rattigan to aid the Sontaran conquest of Earth. He had an ambition to personally kill the Tenth Doctor on meeting him. (TV: The Sontaran Stratagem)

Staal remained in the Sontaran flagship when he ordered his subordinate Skorr to attack UNIT. He was contacted by the Doctor shortly after claiming the Doctor's TARDIS as his trophy. After his plan of turning Earth into a cloning planet for more troops failed, he furiously decided to destroy Earth by force.

Rejecting the Doctor's offer to let him leave Earth alone or be incinerated, Staal berated him. He and all but one of his troops were then killed by Luke, who gave his life to destroy the flagship. (TV: The Poison Sky)

According to one account, Staal survived the flagships destruction and was later encountered again by the Tenth Doctor when he attempted to invade a far-off planet for their sausages, which he and his troops believed to be the best in "the whole of the universe". After he saved Staal from an Ogron, the Doctor convinced him that he didn't need to invade the planet to continue enjoying their sausages, but rather just stop by for lunch. Realising he was right, Staal accepted the Doctor as a friend, and the two of them sat down for lunch with Staal's platoon. (PROSE: Dr. Tenth)

Personality
Staal possessed a personality that was typical for a Sontaran. He loved warfare and was unafraid of death, even berating the Doctor when the Time Lord threatened to blow up his ship. He regretted his race did not fight in the Last Great Time War, which he considered the universe's greatest war.

Staal was also manipulative and treacherous, having manipulated Luke Rattigan into helping him by saying that he would take him and some other teenagers to another planet to rebuild the human race, when he was actually planning to use him as target practice when he was no longer needed.

Staal showed no respect for humanity, claiming that "[Sontarans] say the same of humans" when Luke Rattigan asked how Staal and Skorr told each other apart and called them "stinking human beasts". He was stunned when Luke gave his life to stop them, his last expression being one of incredulity. (TV: The Sontaran Stratagem)

Behind the scenes

 * General Staal and his invasion of modern London is mentioned by Commander Strax in the fourth-wall-breaking cinema intro for The Day of the Doctor. He is disappointed to learn that the anniversary feature is about the Doctor as opposed to a Sontaran historical drama featuring heroic accounts of their various military campaigns which he was anticipating with his clone batch.