Eternal Dalek

The Eternal Dalek was one of the new species of Dalek that emerged from the Progenitor in 1941. It worked some of the machinery inside the Dalek ship and escaped through the Time corridor with the Supreme Dalek, Strategist Dalek, Drone Dalek and Scientist Dalek. Its precise function was unknown. (DW: Victory of the Daleks)

When the Daleks joined The Alliance formed to imprison the 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)

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.