Remembrance of the Daleks
During the Dalek civil war between the Imperial Daleks, led by Davros, and the Renegade Daleks, led by the Supreme Dalek the Special Weapons Dalek was used by Davros against the Renegades. He was able to gain the Imperial Daleks a heavy advantage, and when it was done with it's job it was taken back to the Dalek mother ship.
Asylum of the Daleks
At some point the Special Weapons Dalek was deemed insane and was sent to the Dalek Asylum. Since it was not shown moving or active, it is implied that it is one of the inactive or catatonic Daleks. Since the Asylum was subsequently destroyed it is assumed that he was destroyed with it.
The Witches Familiar
Despite its apparent destruction, the Special Weapons Dalek somehow survived once again and appeared in Skaro's Dalek City, along with various other Dalek models. However, it's fate after the city's collapse remains unconfirmed.
It looks like a normal Dalek however instead of the usual eyestalk and weaponry it has a giant gun with a more powerful laser that causes a greater amount of destruction.