Truth Field
The Truth Field was a field in the town of Christmas on the planet Trenzalore . It caused the inhabitants of the town to tell the truth at all times, and could be the cause of embarrassment. It was said to be stronger and more powerful the closer one was to the tower.
It was later revealed to be the work of the Time Lords, acting through a crack in time and space inside the tower. Seeking to return to the universe, but not wanting to risk harm to themselves, they created a scenario where all of the universe would hear a message they couldn't translate and be drawn to Trenzalore. That message, eventually translated, would ask simply: "Doctor Who"? If the Doctor answered, and did so truthfully due to the truth field, the Time Lords would know it was safe to return to the universe through the crack.
