The Mire

The Mire augment their abilities with stolen technology from every race they conquer. They are heavily armoured with tech such as plasma cannons, holographic heads up displays and scanners, most of which is crammed into their helmets...

But tech isn’t the only thing they steal from their conquests - by extracting testosterone, adrenaline, and other powerful human hormones from the Vikings, the Mire create ‘nectar’ (or ‘warrior juice’ as Clara calls it) which they carry in clear vials on their helmets and consume at high points of battle. Their helmets are therefore their biggest strength, and their biggest weakness.

The Mire’s own technology ultimately saves Ashildr’s life; the Doctor reprogramming a Mire battlefield medical kit to repair Ashildr, but it will never stop repairing her, making her functionally immortal.