Trenzalore

Trenzalore was a planet which, in an alternate timeline, hosted a battlefield graveyard and the tomb of the Doctor. At least before the Siege of Trenzalore, it hosted a small village that according to Tasha Lem was little more than a farming community.

The size of each gravestone of the cemetery was proportionate to the rank of the buried soldier. (TV: The Name 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 told the Doctor 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 the Last Great Time War would restart 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.

Eventually 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.

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 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 an entity that contained the Doctor's personal time tunnel from Gallifrey to Trenzalore. The Great Intelligence invaded this tunnel at the cost of its life to interfere with the Doctor's victories, unravelling everything he had done to protect the universe and beyond. Clara Oswald jumped inside the tunnel as well in order to reverse the damage, though it splintered her into thousands of other echoing versions of herself. After she fixed his timeline, the Doctor leapt into the tunnel to rescue the true Clara, where he reunited with her. 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)

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