Dalek Asylum

The Dalek Asylum was a snowy and mountainous planet used by the Daleks as a prison and "dumping ground" for those among them who had malfunctioned, gone insane and/or become mentally scarred by battles. The Dalek Asylum was protected by a force field and a nanocloud, which converted beings that did manage to enter into Dalek puppets. The sane Daleks left their insane fellows in the Asylum rather than kill them because they epitomised the Dalek concept of beauty: pure hatred. Though the Daleks did not keep a precise count of how many of their brethren are incarcerated on the planet, they suspected it to be in the millions.

Residents
Even amongst the deranged Daleks, there was still an area described as "intensive care", which housed several Daleks who had survived conflicts with the Doctor on planets such as Spiridon, Kembel, Aridius, Vulcan, and Exxilon.

The Daleks of the Asylum came in multiple casings from across Dalek history. (TV: Asylum of the Daleks) Among them were numerous bronze Daleks, (TV: Dalek et al.) their contemporary Black Dalek, (TV: Army of Ghosts et al.) a silver Skaro City Dalek, (TV: The Daleks) an Elite Guard Dalek as seen at the start of the Dalek Civil War, (TV: The Evil of the Daleks) and a silver Dalek as seen on Exxilon. (TV: Death to the Daleks)

Grey Daleks were also present, including one as seen on Spiridon, (TV: Planet of the Daleks) two identical to the first Mark III Travel Machines unveiled by Davros, (TV: Genesis of the Daleks) two as seen following the Dalek-Movellan War, (TV: Resurrection of the Daleks) two Renegade Daleks, an Imperial Special Weapons Dalek, (TV: Remembrance of the Daleks) a white Supreme Dalek and a Strategist Dalek of the New Dalek Paradigm. (TV: Victory of the Daleks et al.)

Creation
The Asylum existed for many years and became the subject of legends, which the Doctor heard. (TV: Asylum of the Daleks)

By an account of the Dalek Wars given by human historians, during the height of the conflicts, the Dalek calling itself Rusty had destroyed a Dalek command ship after an encounter with the Twelfth Doctor had caused him to begin hating the Daleks. When he was brought before the Supreme Council, they found his hatred too beautiful to extinguish and placed him in the Dalek Asylum. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) By another account, Rusty's crusade against its kind was part of post-Time War Dalek history, beginning at a point after the destruction of the Asylum. (PROSE: Dalek Combat Training Manual) Rusty would later resurface on Villengard. (TV: Twice Upon a Time)

Indeed, the concept of the Dalek Asylum and its residents existing in pre-Time War history (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) contradicted the claims made by the Cult of Skaro, who believed themselves to be the last four Daleks in existence after the Last Great Time War (TV: Daleks in Manhattan) before their race rebuilt. (TV: The Stolen Earth, Victory of the Daleks et al.) Additionally, the Asylum planet was covered in the nanocloud; the same historians who claimed the Asylum existed in the Dalek Wars claimed the nanocloud was an invention of the post-Time War New Dalek Paradigm. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

While Daleks from before the Time War undoubtedly existed within the Asylum, the Daleks of the New Paradigm and Resurrected Dalek Empire had access to time travel, providing a possible explanation. (TV: Asylum of the Daleks) While pre-Time War Dalek history was generally locked away from post-War individuals, (PROSE: Prisoner of the Daleks) those same human historians found evidence that the New Paradigm had found access into the Imperial-Renegade Dalek Civil War and interfered with it. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe) Indeed, Renegades and an Imperial Special Weapons Dalek were present in the Asylum. (TV: Asylum of the Daleks)

Destruction
The Alaska was able to breach the Dalek Asylum's force field and crashed into the planet. Junior Entertainment Manager Oswin Oswald was fully converted into a Dalek by the inmates, while the rest of the crew were converted into Dalek puppets by the Asylum's nanocloud. The Asylum was originally fully automated, but over the course of a year, Oswin was able to reduce the Asylum to a ruin by hacking into its systems.

A year after the Alaska crashed on the Asylum planet, the Parliament of the Daleks feared the insane Dalek inmates could escape. In order to destroy the planet and the Daleks inhabiting it, they kidnapped the Eleventh Doctor, Amy Pond, and Rory Williams, and sent them to the surface (by way of a gravity beam) in order to deactivate the planet's defences, as no Dalek was willing to infiltrate the Asylum themselves due to fear of the insane Daleks.

On the planet, the Doctor, Amy and Rory were guided by Oswin who was unaware of her status as a Dalek. Due to her hacking, Oswin could lower the Asylum's force field, but refused to do so until the Doctor rescued her from the escape pod she believed she was trapped in. The Doctor eventually reached the intensive care unit where the Daleks, who were normally completely catatonic, woke up in his presence due to their hatred of him. Before the Daleks could kill him, Oswin hacked the Dalek Pathweb and erased all memory of the Doctor from the Daleks to his shock as even he couldn't hack the Pathweb. Moments later, he learned of Oswin's conversion after finally finding her and she nearly gave into her programing in her despair over the situation. Breaking down, Oswin regained control and decided to end her own existence and that of the Daleks. Oswin lowered the Asylum's force field and ordered the Doctor to run and remember her.

As the Dalek fleet fired on the defenceless Asylum, the Doctor was able to use a teleporter to teleport himself, Amy and Rory aboard the TARDIS on the Dalek ship in orbit. Once the Asylum defences were down, the Daleks fired missiles at the planet, blowing it up to kill the Daleks trapped there. To the Doctor's surprise, when he emerged from the TARDIS to taunt the Daleks after they detected his teleport, Oswin erased the knowledge of his existence from the totality of the Pathweb, not just among those in the Asylum. (TV: Asylum of the Daleks) Memories of the Doctor were restored during the Siege of Trenzalore. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe; TV: The Time of the Doctor)

Alternative timeline
When the Great Intelligence invaded the Doctor's time stream on Trenzalore to turn all of his victories into defeats, the Dalek Asylum was one of the many places where the Doctor died. When Clara Oswald followed him into the time stream, it was revealed that Oswin Oswald was one of many versions of Clara scattered across the Doctor's history with the purpose of saving his life. (TV: The Name of the Doctor)

Behind the scenes

 * DWFC TLV 1 states that the Resurrected Dalek Empire was given a "refined" sense of hatred by the Prime Minister of the Daleks, explaining that it was only after the Prime Minister came to power that the Dalek saw hate as beauty. This would support the idea that the Dalek Asylum was not always a part of Dalek history, but was retroactively inserted into it, explaining how the Cult of Skaro can have thought they were the last of their race: from their perspective, the Asylum had yet to exist.