Trenzalore

Trenzalore was a Level 2 human colony planet, from which the Time Lords broadcast a message through the last of the Cracks in Time. It was home to many human villages, among them the snow-farming village of Christmas that became the site of the Siege of Trenzalore, (TV: The Time of the Doctor) and in an alternate timeline, a battlefield graveyard which included the Doctor's tomb. (TV: The Name of the Doctor)

For 900 years, (PROSE: Tales of Trenzalore) it was the epicenter of a longstanding siege. Trenzalore was attacked relentlessly by many alien races who opposed the return of the Time Lords, while the Eleventh Doctor made his last stand against them all. (TV: The Time of the Doctor)

History
Approximately 150 years before the Eleventh Doctor began his residency on Trenzalore, humans colonists settled on the planet. Some of the first colonists were Jalen Fellwood and Roland Treece, both of whom became headsmen at a time when the Truth Field did not yet exist. (PROSE: The Dreaming) The early settlements stockpiled boronite, which they used to reach underground hot springs by exploding the bedrock that covered them. The leftover boronite was eventually shelved. (PROSE: Strangers in the Outland)

Jalen Fellwood eventually took Summerly Treece as his wife and sacrificed her to evil beings that he worshiped, hoping to get untold powers in return. Roland enlisted a gang of "righteous men" to hunt down Jalen, kill him, and bury his remains in an unmarked grave on salted ground. The truth about Jalen's skeleton was kept a guarded secret throughout the generations and known only to three people. The third was Vida Clatterly, the oldest person on Trenzalore besides the Doctor, and the keeper of Trenzalore's oral history. Nine hundred years after Jalen's death, when it became clear that something was using his bones to harm residents of Trenzalore, Vida finally revealed the story to the Doctor. (PROSE: The Dreaming)

Dorium Maldovar alerted the Doctor to the fact that he would visit Trenzalore at some point in his travels, at "the Fall of the Eleventh". Maldovar also said to him that at this time and place "no living creature can speak falsely or fail to answer." Here, the First Question in the universe would be asked. (TV: The Wedding of River Song)

The circumstances behind the Fall of the Eleventh were revealed when the Doctor was drawn to Trenzalore by an endlessly repeating message emanating through all of time and space. Trapped in a pocket universe, the Time Lords were using a residual crack in time located on Trenzalore to send the message, which translated as "Doctor Who?" through all of time and space. Along with the Doctor, their transmission drew "half the universe" to the planet including the Daleks, Cybermen, Judoon, Silurians, Weeping Angels, the Sontarans, and the Papal Mainframe. The Mainframe arrived first and established a force field around the planet, preventing anyone from approaching and creating a stalemate which everyone was afraid to break due to the fear established by the message.

Teleporting to Trenzalore from the Papal Mainframe, the Doctor found the crack and realised what was going on. After learning what planet it was, the Doctor stayed on Trenzalore so that it wouldn't be destroyed, not willing to restore the Time Lords because a new time war would start due to the presence of so many enemies. For three hundred years the Doctor protected the planet from small incursions by various enemies with the help of the now-renamed Church of the Silence who dedicated themselves to stopping chaos from happening.

Three hundred years later, the force field protecting the planet fell and the Doctor's enemies attacked in force. The Doctor spent centuries protecting the planet with the help of his former enemies, the Silence.

In time, all but the Daleks burned or retreated and the Doctor grew old, frail and forgetful. Tasha brought the Doctor's companion Clara Oswald to Trenzalore so that he wouldn't die alone as he was out of regenerations and was near death from old age. During a final attack by the Daleks on Christmas, the Doctor decided to surrender himself as demanded. But as he stood atop the Clock Tower, ready to accept his final death, the Time Lords, at Clara's request, granted him a new cycle of regenerations. Using the energy from his regeneration, the Doctor destroyed the Dalek forces assailing Trenzalore. Having done their part in ensuring the Doctor's survival, the Time Lords finally sealed the crack linking Gallifrey and Trenzalore, cutting themselves off from the universe to await another chance at returning. Meanwhile, with the siege over and the planet no longer in danger from his enemies, the Doctor, finally free of his obligations to both the Time Lords and the people of Christmas, departed from Trenzalore on a permanent basis and regenerated into his twelfth incarnation. (TV: The Time of the Doctor)

Alternate timeline
In an alternate timeline, the Time Lords never gave the Doctor another regeneration cycle. In this timeline, the Doctor died on Trenzalore and was buried in a giant tomb made out of his dying TARDIS. The tomb was surrounded by a battlefield graveyard containing the fallen from the Siege of Trenzalore, with the size of each gravestone proportionate to the rank of the buried soldier. The planet itself became a desolate wasteland covered with molten cracks, and without its original rings or moons, all of them destroyed. (TV: The Name of the Doctor)

The Doctor was eventually forced to go to this timeline's version of post-siege Trenzalore when the Great Intelligence kidnapped a number of his friends. The TARDIS resisted landing on the planet as the Doctor visiting his own tomb was likely to create a paradox and stopped in Trenzalore's orbit. The Doctor then turned off the TARDIS anti-gravity systems to force a crash landing on the planet's surface, leaving one of its windows cracked from the impact. There he saw his tomb, which was the TARDIS itself. However, this version of his TARDIS was in the process of "dying". It had become quite dilapidated and inflated to giant proportions due to the breaking of its dimension dampeners, resulting in a "size leak" phenomenon. It was here that the Doctor came across the Whisper Men and the Great Intelligence taking the form of Walter Simeon. The Great Intelligence attempted to force the Doctor to open his tomb, his real name being the key. River Song, having manifested outside of the Library, spoke his name without being heard and allowed the Great Intelligence entrance.

Inside the tomb was not a body, but a complex tear in the fabric of space-time that enabled entry into the Doctor's time stream. The Doctor described it as "the scar tissue of my journey through the universe; my path through time and space, from Gallifrey to Trenzalore." The Great Intelligence entered the scar at the cost of its own life to interfere with the Doctor's victories, unravelling everything he had done to protect the Universe, killing all thirteen of his incarnations simultaneously in the process. Clara Oswald jumped inside the scar as well in order to reverse the damage, though doing so splintered her into thousands of "echoes" of herself. Paradoxically, it was his encounters with those splinters that led the Doctor to meet and travel with Clara in the first place. After she fixed his timeline, the Doctor leapt into the scar himself to rescue the original Clara, risking the collapse of his entire time stream to do so. However, they encountered an incarnation of the Doctor that the Doctor claimed had dishonoured his chosen name in his actions. (TV: The Name of the Doctor)

Astronomical data
From space Trenzalore looked blue with little visible atmosphere. It resembled a gas giant despite being a terrestrial planet. (TV: The Time of the Doctor) During the time the Doctor visited his tomb there, in an alternative timeline, Trenzalore looked like a "burning planet" with a highly visible atmosphere with rivers and oceans of lava. These changes were presumably a result of the planet being ravaged by the Daleks during that timeline's version of the Siege of Trenzalore, in which the Doctor was never given more regenerations and instead died on the planet, leaving it to ravaged by the victorious Daleks. (TV: The Name of the Doctor)

Trenzalore seemed to have a similar atmospheric composition to that of the Earth, as humans lived there with no apparent technology. (TV: The Time of the Doctor) Trenzalore originally had rings and two moons, Soror and Frater, which were later lost in the alternative timeline. (PROSE: Strangers in the Outland) Daylight in the village of Christmas only lasted a few minutes, and snow there was common. (TV: The Time of the Doctor)

Trenzalore was of no strategic value to the Nestene Consciousness as it was almost entirely unpolluted, with no toxins, chemicals, acids, metals or smoke in the air. The Doctor was the only reason it had to breach the planet's technology barrier. (PROSE: Strangers in the Outland)

Known geographic features
Christmas was a town inhabited by humans, (TV: The Time of the Doctor) where snow was farmed. (PROSE: Let it Snow) Beyond Christmas, other sites included the Outland, Devil's Elbows Canyon, the Goat Path, Lake Lagda, Fafnir's Crest, the Choke, (PROSE: Strangers in the Outland) the Glade of Everdell and Preacher's Clump. (PROSE: Let it Snow)

Other references
After having saved Gallifrey from the Last Great Time War, at the insistence of the Tenth Doctor, the Eleventh Doctor revealed he had visited his tomb on Trenzalore. His predecessor commented he didn't want to go there. (TV: The Day of the Doctor)

 Trenzalore