Staal the Undefeated was a Sontaran, and General of the Tenth Sontaran Battle Fleet during the invasion of Earth in 2009. He had a great love of warfare and regretted that his race were not involved in the Last Great Time War, which he considered to be one the universe's greatest wars. He forged an alliance with Luke Rattigan to aid the Sontaran conquest of Earth. He had an ambition to personally kill the Doctor upon meeting him, partially due to the Doctor's enmity with the Sontaran race and Staal's grudge over not being allowed to fight in the Time War. (DW: The Sontaran Stratagem)
I am General Staal of the Tenth Sontaran Battle Fleet, better known as Staal the Undefeated.
Staal remained in his battle ship as he order Skorr to attack U.N.I.T. He is contacted by the Doctor shortly after claiming the TARDIS as his trophie. After his plan of turning Earth into a cloning planet for more troops fails, he in fury orders his army to go back to tradition and destroy Earth by force, but is confronted by the Doctor who offers him a chance to leave, or be incinerated. Stall merely mocks at this and is ready to have him shot but Luke unexpectedly appears and sends the Doctor back into the teleport pod, while he destroys the Sontaran Warship with Staal and the remainder of his troops on it. (DW: The Poison Sky) Stall is without dought the WORST sontaran ever