This spoiler comes from GBase. It was a long post but I'll summarize:
There is a bell on a planet that rings out a distress call to all races on every planet in the galaxy. People are afraid to answer it, but the Doctor does because he is curious. When he arrives he encounters the angels that touch his leg and he is injured. He is able to escape but later loses his leg due to it turning into stone. He goes to a small town on the outskirts of a town called Christmas which sits in a field of truth (Remember the warning about Trenzalore..."where no creature can speak falsely" line) and realizes he is on Trenzalore pre the Name of the Doctor episode. He learns that the area is surrounded by many foes: Daleks, The Silence, Cybermen, Angels, and Sontarans. They all want to attack but they are afraid because they know the Doctor is there.
Since he knows that this would take a lot of time to resolve, he sends Clara back with the Tardis (Kind of like what the 9th did with Rose in Bad Wolf) for her safetey which strands him on Trenzalore. He teams up with the Papal Mainframe and repels enemies for 300 years until the Daleks remain. The Doctor stays in a stalemate with the Daleks for 600 more years until he explains that he is getting old and he is at the end of his life and has no more regenerations left.
The Doctor then confronts the Daleks and tells them to just kill him. Right before they kill him a crack of light appears and the energy causes the Doctor to regenerate into Capaldi.
So from the above it would appear that the Doctor at the end via Deus Ex Machina is given a new cycle of regenerations which I guess comes from the Time Lords on Gallifrey after they are found sometime in the future