Trenzalore

Trenzalore was a Level 2 planet from which the Time Lords broadcast a message through the last of the Cracks in Time. It was home to a single human colony, the village of Christmas, described by Tasha Lem as little more than a farming community. Christmas became the site of the Siege of Trenzalore, and in an alternate timeline, a battlefield graveyard which included the Doctor's tomb. (TV: The Name of the Doctor, TV: ''The Time of the Doctor)

History
According to Dorium Maldovar, the Doctor would visit Trenzalore at some point in his travels, which would coincide with "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.

The Silence were determined that the Doctor never reach Trenzalore, so that the question would never be answered. It was a creed of the Silence that "Silence will fall when the Question is asked", or as Maldovar translated it, "Silence must fall". (TV: The Wedding of River Song)

Trapped in a pocket universe, the Time Lords used a residual crack in time located on Trenzalore to send a message through time and space: "Doctor Who?" which drew "half the universe" to the planet including the Daleks, Cybermen, Judoon, Weeping Angels, the Sontarans, the Eleventh Doctor 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 realized what was going on. After learning of what planet it was, the Doctor tricked Clara home and remained to protect 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. A faction led by Madam Kovarian broke off from the Church to try to stop these events but only ended up causing them by creating the cracks in the first place.

Eventually the TARDIS returned with Clara, but the Church had been turned into Dalek puppets. Though the Doctor and Clara escaped with Tasha's help, the force field protecting the planet fell and the Doctor's enemies attacked in force. The Doctor sent Clara home once again and spent centuries more 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 Clara back so that he wouldn't die alone as he was out of regenerations and was near death of old age. During a final attack by the Daleks on Christmas, the Doctor decided to surrender himself as demanded, but as the Daleks prepared to kill him, the Time Lords, at Clara's request, granted him a new cycle of regeneration. The Doctor started to regenerate and used the energy from it to destroy the Dalek forces assailing Trenzalore. As the Time Lords closed the crack after saving the Doctor, The Doctor and Clara left Trenzalore once and for all and the Doctor regenerated into the Twelfth Doctor, changing the future where he died on Trenzalore. (TV: The Time of the Doctor)

Alternate timeline
Instead of regenerating, the Doctor died on Trenzalore and was buried in a giant tomb made out of his own 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.

The Doctor was eventually forced to go to 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 forced 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 "scar": a complex tear in the fabric of spacetime resulting from the Doctor's travels 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. Clara Oswald jumped inside the scar as well in order to reverse the damage, though it splintered her into thousands of "echoes" of herself. After she fixed his timeline, the Doctor leapt into the scar to rescue the original Clara. However, they encountered an unknown 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 and it resembled a Gas giant despite being a terrestrial planet. In its alternative timeline Trenzalore looked like a "burning planet" with a highly visible atmosphere with rivers and oceans of lava. Trenzalore seemed to have a similar atmospheric composition to that of the Earth as Humans lived there with no apparent technology. Trenzalore originally had rings and two moons, which would later be lost in the alternative timeline. Daylight on Trenzalore only lasted a few minutes which could be due to an unusual orbit or axial tilt. Snow is common on Trenzalore suggesting that it is far away from its star.

Towns

 * Christmas

Sites
(All in alternative timeline)
 * The Doctor's tomb
 * River Song's fake tomb (Alternate entrance to The Doctor's tomb)
 * Various other graves and tombs

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 their tomb on Trenzalore. His predecessor commented he didn't want to go there. (TV: The Day of the Doctor)

Trenzalore