Eternal Dalek

The Eternal Dalek was one of the new species of Dalek that emerged from the Progenitor in 1941. It was a special Dalek, like the Supreme Dalek, as that there was only ever one Eternal. (DWA: 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. The new Daleks deposed the old ones and attempted to destroy Earth. 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 unknown. (DW: Victory of the Daleks)

When the Daleks joined the Alliance, formed to imprison the Eleventh Doctor in the Pandorica in order to save the Universe, they arrived at Stonehenge, 102 A.D. along with the rest of the Alliance. The Eternal, along with the Supreme and a Drone, witnessed the Doctor being locked in the Pandorica. (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 scenario would later be undone by the Doctor. (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 Eternal Dalek is notably the only one of the five Progenitor Daleks to have black orbs on its design rather than silver ones.