Atraxi

The Atraxi were a sophisticated alien police race, which formed a galactic police force. They were very efficient with their work, tracking down any prisoners who escaped their facilities and executing them. However, they were not the most intelligent of aliens; one failed to notice overloading technology in Leadworth while scanning for Prisoner Zero.

Biology
The appearance of the Atraxi themselves is impossible to know, since they were never actually seen outside of their giant eyeball, which was capable of seeing and speaking. The Atraxi ships had a rather large crystalline structure, much like a snowflake. They were able to travel both in space and in atmosphere. (TV: The Eleventh Hour)

Abilities
The Atraxi could remotely activate and control electronic media on Earth, allowing them to send a message in every human language, through every available device. They tracked the media, pinpointing a worldwide computer virus to its origin and learn the planet's history. Though capable of scanning the planet on a worldwide scale, they could not find multi-forms when shapeshifted into an unknown form. The Atraxi fleet was capable of incinerating a planet (in Earth's case, requiring twenty minutes to charge) and creating a force field to seal the planet's atmosphere for its destruction. The Atraxi ship which came directly to Earth could also project holograms. (TV: The Eleventh Hour)

History
The Atraxi attended the annual Space-Time Police Officers' Ball on 18 January along with the crew of the Teselecta and the Shadow Proclamation, meeting to eat some vol-au-vents and share stories of their greatest assets. (PROSE: Time Traveller's Diary)

In 102, the Atraxi were among the races who joined the Pandorica Alliance. They came to Stonehenge and helped imprison the Eleventh Doctor in the Pandorica in order to save the universe. (TV: The Pandorica Opens)

The Atraxi ran an extradimensional prison which contained Prisoner Zero. When Prisoner Zero escaped through a crack in space in 1996, they tried to find it. Twelve years later, they were able to trace it to Earth. They threatened: "Prisoner Zero will vacate the human residence or the human residence will be incinerated! Repeat!".

Though the Eleventh Doctor tried to attract their attention by activating alien technology, his sonic screwdriver burnt out before they noticed. Eventually, he created a computer virus to send the Atraxi a message and got Jeff Angelo and a team of geniuses to distribute it worldwide. The message was '0'. The Atraxi tracked the virus to a mobile phone held by the Doctor. When the Doctor and Amy Pond trapped Prisoner Zero in its own form, the Atraxi were able to locate and restrain it. Though the Atraxi attempted to leave at this point, the Doctor called them back for breaking Article 57 of the Shadow Proclamation, showing them his past incarnations' victories and telling them to never return to Earth. (TV: The Eleventh Hour)

Even as late as the 52nd century, stories of the Atraxi incident and the Doctor calling them back for a scolding were still told, the Thin One and the Fat One talking about it prior to the siege at Demons Run. (TV: A Good Man Goes to War)

Luachmhar was attacked by the Atraxi for snowflake diamonds. (PROSE: Anything You Can Do)

Behind the scenes

 * In cut dialogue from The Girl Who Waited, Amy was to have been offered "Atraxi cuisine".
 * In the original script for The Eleventh Hour, the Doctor referred to the Atraxi as a "level 20 civilisation".

Atraxi