The Atraxi are a peculiar species of large sapient beings whose outside appearance is similar to a giant eyeball at the center of a crystalline, snowflake-like structure. It appears that they can detach the eye from the crystalline ring that surrounds it, perhaps implying that the Atraxi themselves are just huge eyeballs and the crystalline structure is a symbiotic organism or spaceship. They are able to travel through both planetary atmosphere and space vacuum. The Atraxi are well known as galactic policemen, forming a respected security force to trace back and arrest criminals of different species. They are also one of the member races of the Alliance.
The Atraxi were once referred to as a level 20 civilization, although the exact meaning of this is still unclear. They appear to master extremely powerful technology, which includes being able to control the entire media of Earth, sending messages in all Human languages known in every available device (the Vogons also possess this technology); tracing a computer virus to its exact origin on a planetary scale; projecting holograms; surrounding a planet with a force field and even destroying an entire planet with a weapon that takes only 20 minutes to charge. They can also travel through parallel dimensions; though whether this is a natural trait or a technological achievement remains unclear. They keep the most dangerous criminals in an extra-dimensional prison.
The Atraxi homeworld is unknown, and being spaceborne it is possible that they have none. However, the Doctor once mentioned a planet called Atraxi III, which according to him is inhabited by Giant Mosquitoes.
When the Atraxi threatened to incinerate Earth if the alien known as Prisoner Zero didn't surrender, the Doctor managed to enlist the aid of several others and lead the Atraxi to Prisoner Zero's location in a hospital - allowing the Atraxi to capture the fugitive. However, he then called the Atraxi back due to their threat to incinerate Earth, which violated article 57 of the Shadow Proclamation due to Earth been a 'fully established Level 5 planet'.
The Doctor talks to the Atraxi
The Doctor talks to the Atraxi
Talking to one of the Atraxi on the roof of the hospital which changing into a new outfit, the Doctor (identified as not been human by the Atraxi) questioned both whether Earth was a threat to the Atraxi or if Earth's people where guilty of any crimes by the Atraxi's laws. After receiving a 'No' answer for both those questions, the Doctor then asked if Earth was protected - asking also what happened to the other species who came to Earth with bad intentions. After seeing images of his former lives flash before him, the Doctor (now in a new outfit) introduced himself before telling the Atraxi "basically...run." Heeding the Doctor's words, the Atraxi he was talking to quickly fled.