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, and was used to locate people who were deemed to have committed crimes but had escaped punishment. After finding them near the end of their respective timelines (so as 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 was manned by a staff of four hundred twenty-one people. They could change its appearance to that of any humanoid being. When the Eleventh Doctor, Amy, Rory, and Mels crashed through Adolf Hitler's office, the Teselecta was trying to punish Hitler, but got him in the wrong time period. When they saw Mels, who had regenerated, revealing herself as Melody Pond, they stopped trying to go after Hitler, and tried to punish her, for their records indicated that she would be the one to kill the Doctor. They were thwarted by Melody's parents, Amy and Rory, who deactivated the devices protecing the crew from the antibodies. They were forced to teleport to a spaceship above the Earth, abandoning the Teselecta. The Teselecta was used to show Melody that River Song, the friend of the Doctor and his companions was whom she would later become. The Doctor also extracted data from it regarding his death and had it reveal the Silence's purpose. (DW: Let's Kill Hitler)

The Teselecta began investigating the Silence and was approached by the Doctor for information. After leading the Doctor to someone who could help, the Teselecta was approached again to deliever the Doctor's letters 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 for the Teselecta to take on his shape to fake his own death. When trapped in the alternate timeline, the Doctor was in fact in the Teselecta the whole time and revealed this fact to River Song before sending them back to Lake Silencio. There, River shot the Teselecta, apparently killing the Doctor and allowing him to fake his own death successfully. (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. At some time in the future and on the 22nd of April 2011 the Teselecta took form of a deceased member of the Silence and at his request, The Doctor.

Miniaturisation ray
The Teselecta was equipped with a miniaturisation ray, which it used to miniaturise 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. (DW: Let's Kill Hitler)