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)
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.
Given that Eternal means forever, the Eternal Dalek may be the only one that can activate an Emergency Temporal Shift as a way to revive the Daleks once again should the Doctor find a way to destroy them once again.
Trivia
- The Eternal Dalek is notably the only of the five Progenitor Daleks to have black orbs on its design rather than silver ones.