Ashildr, daughter of Ionhar (Ian Conningham), is introduced in "The Girl Who Died" (2015), where she is portrayed as a Viking puppet maker and storyteller.

Having lived through many wars and suffered many losses, she plans to travel the galaxy with an alien Leonian called Leandro (Ariyon Bakare) after the Doctor helps her retrieve an amulet called the Eyes of Hades, not knowing this will create a portal using the death of rival highwayman Sam Swift the Quick (Rufus Hound).

Ashildr uses her spare piece of Mire medical technology to restore Sam's life and close the portal to Leandro's world.

[4] Before "Hell Bent", in "Heaven Sent", the Doctor mentions that Ashildr/"Me" is going to destroy Gallifrey, because she is the "hybrid", echoing a prophecy related by Davros in "The Magician's Apprentice" / "The Witch's Familiar".

In the ninth series finale "Hell Bent", the Doctor attempts to help Clara cheat death and runs away with her from the planet Gallifrey (set roughly 14 billion years after Ashildr became immortal,[note 2]) to the end of time itself (over 100 trillion years in the future [note 3]).

Clara promises Ashildr she will return to Gallifrey to face her death as history records, but not before seeing the universe in more depth first.

I can't wait to meet the cast and crew and start filming, especially as we'll be shooting not too far from my home town.

The Doctor has now officially created a functional immortal out of a normal human, and given how he's held up with such a life span, it looks like things are about to get scary.

"[9] Sam Warner of Digital Spy reported, when talking about the character's debut appearance, that "Opinion was mostly positive, though Ashildr hasn't convinced everyone just yet.