Ashildr is a fictional female Human in the Doctor Who television series who was portrayed on screen by Maisie Williams. The Doctor and Clara first meet Ashildr when the Tardis lands in a Viking village where they go up against enemy robots known as the Mire. When Ashildr is killed, Clara persuades The Doctor to revive her which he does using Mire technology. The unintended consequence of using Mire technology is that Ashildr is now immortal and will live forever.
Fast forward a few hundred years and Ashildr is now a highway woman much like in the way of Dick Turpin, the notorious highwayman of the early eighteenth century who has it by legend that he rode a horse non-stop from London to York without stopping on his horse Black Bess. However, Ashildr does nothing like Dick Turpin. Ashildr now calls herself Me. She lives in a very large house with an alien that looks like a lion. However the line betrays Ashildr.
Fast forward to the present day and Ashildr is now Mayoress Me in a secret road in London where she unexpectedly passes sentence on Clara who faces the raven and is killed.
In Hellbent, the last episode of that series, The Doctor travels to the very end of time where the only person alive is Ashildr who tries to persuade the Doctor to undo much of what he has done for the universe.
The last we see is that Ashildr's space ship is a diner and she is off travelling time and space with Clara. With the rumoured return of Clara for Peter Capaldi's last episode at Christmas, it is probably unlikely that Ashidr will also return.
Note the extra 'i' in the header above. The character Áshildr is a take on the ancient norse names, Áss (with an apostrophe on the 'A') meaning God and Hildr being battle according to the Radio Times. Hildr was also a name of a Valkyrie, one of 12 of Odin's handmaidens who choose which soldiers lived or died on the battlefield. According to Nordic Names, it was an old Nordic Name and not one made up.
In World of Warcraft, in the Stormheim zone, you will encounter a character called Ashildir with the 'i'. Ashildir is a dead Valkyrie who aids you on your levelling quests as you aim to reach level 110.