Skarr

Skarr was a Sontaran Commander who infiltrated Christmas during the Siege of Trenzalore with an another Sontaran. The two Sontarans got into the town with the help of an invisibilty cloak. Unfortunately the cloak was unmasked by the Eleventh Doctor's sonic screwdriver. The two Sontarans were destroyed by the Papal Mainframe soon after the detection. (TV: The Time of the Doctor)