Teselecta

The Teselecta was a Justice Department Vehicle, staffed by a crew that had been miniaturised by a compression field. It could travel through time, and was used to locate people who were deemed to have broken the law but had escaped punishment. After finding them near the end of their respective timelines, the crew would administer their own punishment - usually in the form of some painful torture before death.

The Doctor, Amy, Rory, and Mels encountered the Teselecta in Berlin, 1938 where they accidentally stopped it from killing Adolf Hitler.

History
The Teselecta was manned by a staff of four hundred twenty-one people. They could change its appearence to any being. At the time the Doctor, Amy, Rory, and Mels crashed through Hitler's office, the Teselecta was trying to kill Hitler, but got him in the wrong time period. It wasn't until they discovered Melody Pond (who had regenerated into her third incarnation) that they stopped trying to go after Hitler, and tried to kill her, for their records indicated that she would be the one to kill the Doctor. They were thwarted by Melody's parents, who deactivated the devices that protected the crew against the antibodies. They were forced to teleport to a spaceship above the Earth, abandoning the Teselecta. The Telesecta was used to show Melody who "River Song" was. The Doctor also extracted from data from it regarding his death. (DW: Let's Kill Hitler)

Shapeshifting
The Teselecta could resemble anyone or anything whose likeness it has scanned. During its time in Berlin, 1938, 6018 took on the forms of a janitor, a Nazi official, a Nazi soldier (both 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)

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

Antibodies
The interiors of the Teselecta were defended by the Teselecta Antibodies. Anyone without the proper privileges was killed.