Key to Time

The Key to Time is a powerful and mythic artifact which the Guardians of Time used to maintain control over universal order and chaos.

On at least three occasions, the Doctor has been sent on quests to assemble the key from its secretly distributed fragments.

Creation
The Key to Time was originally created to wipe the memories of the participants of the Millennium War, though it later gained a more general purpose being capable of manipulating the fundamental nature of the universe itself. (PDA: The Quantum Archangel)

The quest
Millennia later, when universal balance was upset, the White Guardian enlisted the Doctor gather the Key's six segments. Further to this purpose, he had enlisted the help of the Time Lady Romana, for whom the White Guardian had misleadingly appeared as the Lord President of Gallifrey. Romana, who the White Guardian had entrusted with a Tracer, would serve as the Doctor's companion during the quest. (DW: The Ribos Operation)

Meanwhile, the Black Guardian used the Shadow to stop the Doctor from obtaining the sixth segment. Once the Doctor had defeated the Shadow, he assembled the Key and outwitted the Black Guardian, enabling the White Guardian to restore universal balance. The Key was then broken apart and scattered throughout the universe again, assuming five new shapes and restoring Princess Astra to her natural form. (DW: The Armageddon Factor)

The quest completed, Romana chose to continue travelling with the Doctor, at one point regenerating and taking on the appearance of Astra.

Sidetracks to the quest
While the Doctor and Romana searched for the Key, they were twice interrupted by outside sources emitting chronometric radiation that disrupted the tracer's ability to work, consisting of a chronometic pulse caused by a breach in the higher dimensions (PDA: Tomb of Valdemar) and a malfuctioning time cabinet (MA: The Shadow of Weng-Chiang).
 * Another adventure that took the Doctor and Romana to Europe in the 1920s has been identified as taking place after the quest was completed (BFA: Stealers from Saiph).

Identities of the segments

 * A lump of Jethrik crystal taken to Ribos by Garron (DW: The Ribos Operation)
 * The pillaged planet Calufrax, now shrunk down to football size by the Zanak. (DW: The Pirate Planet)
 * The former Great Seal of Diplos, kept in the form of a necklace on Earth by Cessair. (DW: The Stones of Blood)
 * A Tara statue. (DW: The Androids of Tara)
 * A holy relic of the Swampies, swallowed by Kroll on a Delta Magna moon. (DW: The Power of Kroll)
 * Princess Astra of Atrios. (DW: The Armageddon Factor)

Search by the Doctor's fifth incarnation
The Doctor was subsequently put on another quest for the six scattered segments, aided by Amy, a Tracer in the form of a young human. The Grace—powerful beings from outside Time who had originally created the Key—had meanwhile sent another Tracer, Zara, in search of the segments, intending for each Tracer to find three segments each to prevent anyone gaining the full power of the Key, only for things to go wrong when Zara was corrupted and convinced to seek the power of the full Key for herself while Amy became increasingly opposed to her original purpose as the Doctor helped her become a person. (BFA: The Key 2 Time - The Judgement of Isskar)

However, Zara joined forces with the White Guardian some time after this, and eventually the Doctor and Amy, giving the completed key to the Doctor, allowing him to destroy it in the Chaos Pool, the place of its creation, thus preventing the Black and White Guardians from using it and simultaneously scattering the Grace—who had personified in the Key—across all of Time, thus preventing any of them from using the Key as he recognised that its power was too great. (BFA: The Key 2 Time - The Chaos Pool

Identities of the segments

 * A compass salvaged from the wreckage of the ship Green Man's Burden on the planet Chaos(BFA: The Key 2 Time - The Judgement of Isskar)
 * The lake of Eratoon (CC: The Prisoner's Dilemma)
 * The capstone of a Ice Warrior pyramid on Mars (BFA: The Key 2 Time - The Judgement of Isskar)
 * The Valdigian castle of Safeplace (BFA: The Key 2 Time - The Judgement of Isskar)
 * Lord Cassim's gold compressed into a single shard (Paradoxically only created by the actions of the powerless Black Guardian as he tried to find it) (BFA: The Key 2 Time - The Destroyer of Delights)
 * Romana/President Astra of Atrios (Originally Romana following her first regeneration, but Amy and Zara joined forces to transfer the Key's essence from Romana to Astra) (BFA: The Key 2 Time - The Chaos Pool

Search by the Doctor's seventh incarnation
Years later, the White Guardian sent the Doctor, Ace and Benny on a quest to find the six segments of the Key to Time, scattered across the Doctor's personal past, and disguised as items connected with him in some way. (DWM: Time & Time Again)

Identities of the segments

 * The Doctor's hat, which Benny obtained from him in the Land of Fiction. (DWM: Time & Time Again)
 * A jelly baby with a missing leg, which Benny took from the Doctor while chased by Mandrels on the stolen section of the planet Eden. (DWM: Time & Time Again)
 * A fencing sword used by the Doctor to parry with Ace in Sir Reginald Styles' mansion. (DWM: Time & Time Again)
 * A cricket ball hit by the the Doctor at Lord Cranleigh's party at at the latters' home. (DWM: Time & Time Again)
 * A cat badge formerly owned by the Doctor's sixth incarnation, reluctantly given to his next incarnation. (DWM: Time & Time Again)
 * The TARDIS instruction manual dropped by the first incarnation of the Doctor as his own seventh incarnation saved his life. (DWM: Time & Time Again)

Nature and functions
Its shape was that of a perfect crystalline cube, comprised of six segments that must be pieced together to gain the full power of the Key. (Although temporary segments could be created if one had enough components of the original Key to temporarily access at least some of its power, but this was dangerous due to the Key's segments being linked on a fundamental level). Cessair of Diplos had found out how to use the segment she had obtained to temporarily appear as Romana, (DW: The Stones of Blood) and another fragment demonstrated enough power to make a normal-sized squid grow to gigantic proportions after it was swallowed.

Other information
At some stage, the Master obtained a segment of the Key to Time and used it to tear apart the constellation of Mandusus. (MA: Cold Fusion)