The Woman (Hell Bent)

A woman found the Twelfth Doctor inside the barn he used to sleep in when he returned to Gallifrey's drylands. She warned the Doctor that the Time Lords would kill him. She later knocked on the barn door to warn the Doctor of the respective arrivals of the General, the High Council and Lord President Rassilon. (TV: Hell Bent)

Behind the scenes
Steven Moffat, the showrunner during the ninth series, has said that he has "no idea" who this woman is supposed to be, and leaves it to "Whovians" to decide whether they want her to be the Doctor's mother, also saying that accepting her as being the same as the Woman from The End of Time is up to them.