The Eternal Dalek was one of the new varieties of Dalek that emerged from the Progenitor in 1941. It was a special Dalek, like the Supreme Dalek. The Eternal was charged with the continuation of the race. Yellow in more than once sense, it would run to the end of time and space to avoid destruction by The Doctor.
The Daleks joined the Alliance formed to imprison the Eleventh Doctor in the Pandorica and save the universe from the cracks in time created from the TARDIS exploding. When they arrived at Stonehenge, 102 A.D. with the rest of the Alliance, the Eternal, the Supreme and a Drone, witnessed the Doctor being locked in the Pandorica. (TV: The Pandorica Opens) The Eternal Dalek was reduced to dust and stone along with the Supreme Dalek and the Drone Dalek following total event collapse. This timeline would later be undone by the Doctor, restoring the Eternal and the other Daleks in their proper place in time, never knowing of the cracks. (TV: The Big Bang).
Doctor Who Confidential revealed that the name of this Dalek was provided by Steven Moffat in a "naming session" with Mark Gatiss. Neither came up with a meaning for the name, but Moffat opined in Confidential that he thought that the functionality of the Eternal would one day become critical to a story. The Dalek Handbook quotes Gatiss as saying "its exact function is a total mystery, but it's probably something to do with the Progenitor device and the continuation of the race."
The Brilliant Book 2011 calls attention to the fact that the Eternal Dalek's colours are usually found on the most dangerous animals in the wild.
The Eternal Dalek is notably the only one of the five Progenitor Daleks to have black orbs on its design rather than grey ones, making it appear somewhat similar in its colour-scheme to the Gold Daleks from TV: Day of the Daleks and Frontier in Space.
The package of the toy figure of Eternal refers to the Dalek as The Eternal.
The Eternal Dalek is the only Dalek of the New Dalek Paradigm that is yet to appear in any material outside of the main TV series, although a yellow Dalek (with the dark grey bumps of the others rather than black) is seen on the loading screens for GAME: City of the Daleks, and in the game itself. It is the only one of the five Dalek variants absent in The Eternity Clock.