You may be looking for Skar.

Skarr was a Sontaran commander who infiltrated the town of Christmas during the Siege of Trenzalore, along with a fellow Sontaran apparently from the same clone batch. The two Sontarans got into the town undetected, having concealed their vehicle with an invisibility cloak. The cloak was unmasked by the Eleventh Doctor's sonic screwdriver. The two Sontarans were then killed by the Papal Mainframe soon after the detection. Apologising for their death, the Church of the Papal Mainframe stated that they had notified a relevant afterlife. (TV: The Time of the Doctor)

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