Eternal Dalek

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. There was only ever one Eternal. (DWAN: Doctor Who The Official Annual 2011)

History
The original Eternal, along with the other original four, the Supreme, Strategist, Drone and Scientist, was created by the Progenitor when the old Daleks tricked the Eleventh Doctor into providing a testimony of his and their idenities to activate it. The new Daleks deposed the old ones as they were impure due to being created from Davros' DNA (see The Stolen Earth/Journey's End) and attempted to destroy Earth with an Oblivion Continuum bomb inside their android, Professor Edwin Bracewell. The Eternal worked some of the machinery inside the Dalek ship and escaped through the Time corridor with the others when their plans failed. Its precise function was never revealed, leaving it unknown for the time being. (DW: Victory of the Daleks)

When 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; 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. A(DW: 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, leaving the Eternal and the other Daleks in their proper place in time, never knowing of the cracks. (DW: The Big Bang).

Behind the scenes

 * 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.
 * The package the toy figure of Eternal refers to the Dalek as 'The Eternal'.