Stone Dalek

The Stone Daleks were a pair of progenitor Daleks, left as an 'after image' when Dalek history was temporarily erased in a world where the universe was destroyed in 102 A.D, destroying all worlds and stars throughout history, except for the Earth and its moon.

History
The stone Daleks were the Progenitor Daleks present at The Alliance's sealing of the Doctor in the Pandorica in 102 A.D. As the cracks in time continued to run rampant across the Universe, Dalek history was erased, leaving the stone Daleks as 'after images'. The Pandorica contained atoms from the original universe, from before the cracks started to erase events, and had the ability to sustain life.

Light from within the Pandorica restored one of the Daleks, albeit with limited power. The Dalek shot the Doctor, managing to severely wound the Time Lord, but at the cost of depleting its energy reserves. It was then confronted by River Song. Initially disbelieving that River would destroy it, based on the fact that she was one of the Doctor's companions, after checking it's records and discovering who she was. The Stone Dalek, realising River could shoot it through the eyestalk with an alpha meson gun, killing it, begged for mercy and was unsuccessful.

When the Doctor reset the universe and timeline, the Stone Daleks, along with all things that were destroyed throughout history, were restored to their original form and place in space and time. (DW: The Big Bang)

Behind the scenes

 * The reactivated Dalek was the first to actually be heard crying "Mercy". The lone Dalek in DW: Dalek cried for the Doctor to "Have pity" (as did Davros in DW: Genesis of the Daleks and DW: Remembrance of the Daleks).
 * It is unknown which Dalek of those present under Stonehenge the restored Dalek actually is. The lack of a deep voice does suggest that it is not the Supreme. Its longevity may mean that it was the Eternal. However, it's voice is similar to those of the Red Drone Daleks in City of the Daleks and Nicholas Briggs has stated that the voice used in the Adventure Games is the voice he would use if he ever had to voice the Red Daleks on TV.
 * This is not the first time Daleks have been seen in a museum. A Dalek was on display in the Space Museum (DW: The Space Museum) and one Dalek was kept in The Cage in van Statten's underground Musuem (DW: Dalek)