Teselecta

The Teselecta was a Justice Department vehicle staffed by a crew who had been miniaturised by a compression field. It could travel through time. It was used to locate people deemed guilty of crimes who had escaped punishment. After finding them near the end of their respective timelines (to avoid changing history), the crew would administer their own punishment - usually in the form of some non-lethal, painful torture, so that afterwards they could die in the manners recorded by history.

History
The Teselecta had a staff of four hundred twenty-one people led by Captain Carter. They could change its appearance to that of any humanoid being. When the Eleventh Doctor, Amy, Rory and Mels crashed the TARDIS into Adolf Hitler's office, the Teselecta was trying to punish Hitler, but got him too early. When they saw Mels, who had regenerated, they recognised her as Melody Pond and stopped their pursuit of Hitler to punish her instead; their records showed she would kill the Doctor. They were thwarted by her parents, who turned off the devices protecing the crew from the ship's antibodies. They teleported to a spaceship above the Earth, abandoning the Teselecta, which was used to show Melody that she would become River Song, a good friend of the Doctor and his companions. This helped Melody to resist her childhood programming as the Silence's assassin. The Doctor also had it reveal the Silence's purpose and extracted data from it regarding his death. (DW: Let's Kill Hitler)

The Teselecta was later recovered by its crew. While investigating the Silence by posing as Gideon Vandaleur, a deceased envoy of the Silence, the Doctor approached the Teselecta for information. After leading the Doctor to someone who could help, the Teselecta was again approached to deliver the Doctor's mail lest he cross his own timestream. The Captain asked the Doctor if there was anything else the crew could do to help, as they really did believe in justice and wanted to help, despite the Doctor's contempt. The Doctor asked the crew of the Teselecta to help him fake his own death. (DW: The Wedding of River Song)

Shapeshifting
The Teselecta could resemble anyone or anything whose likeness it has scanned. During its time in Berlin, 1938, it took on the forms of a janitor, a Nazi official, a Nazi soldier (with and without a motorbike), as well as Amy Pond and River Song. A member of the crew stated that it had once taken the form of Rasputin, but with green skin (DW: Let's Kill Hitler). At some time in the future the Teselecta took the form of Father Gideon Vandaleur, a deceased member of the Silence, and at his request on 22 April 2011, the Eleventh Doctor. (DW: The Wedding of River Song) It was even able to mimic regeneration at Lake Silencio. (DW: The Impossible Astronaut) The Teselecta had a lot of damage resistance, as the Doctor declared that he "barely got singed" when his 'body' was being cremated at Lake Silencio. (DW: The Wedding of River Song)

Miniaturisation ray
The Teselecta was equipped with a miniaturisation ray, which it used to shrink people and transport them into the Teselecta. Compression fields were used to keep people miniaturised.

Antibodies
The interiors of the Teselecta were defended by robotic organisms called "the Antibodies". Anyone without the proper privileges was killed, such as the Nazi Zimmerman in 1938. (DW: Let's Kill Hitler)

Alternate Universe
The Teselecta, still imitating The Doctor, was sent to an alternate universe where all of time was happening at once when River Song refused to shoot it, believing it was The Doctor. It found The Doctor's companions and went to Area 52 with them. The Doctor realised that if the Teselecta came into contact with River, time would be restored. This happened, allowing time to continue as normal. (DW: The Wedding of River Song)