Doctor Who (2005) Season 9 Episode 5 The Girl Who Died

The Doctor and Clara land in the middle of a Viking village, only to find that the warriors have been taken over by an alien race known as the Mire. The villagers are outmatched, outnumbered, and outgunned - until the arrival of a new warrior, a girl named Ashildr. However, the Doctor knows that victory will only be temporary unless he can figure out a way to make Ashildr immortal. As the Mire draw closer, the Doctor is forced to make a desperate gamble, setting up a showdown with an enemy that can only be defeated through cunning and sacrifice.