Mire

The Mire were an advanced warrior race from the future. According to the Twelfth Doctor, they were considered to be one of the deadliest in the universe.

History
They were fed by stamina, strength, adrenalin and testosterone, and came to a Viking village on Earth to harvest the strongest warriors. Their leader took the appearance of the Norse god Odin to play on the Viking's beliefs. He invited their strongest warriors to dine with him in Valhalla, whereas in reality they sapped their strength and killed them in the spaceship. However, they spared Ashildr and Clara Oswald because of their possession of the sonic sunglasses.

Foolishly, Ashildr declared war on the Mire, and their leader promised to bring down ten of his best warriors to fight. The Mire beamed Ashildr and Clara back to the village, and Ashildr told the village of their impending doom, for the Mire had already taken their warriors.

Later, "Odin" arrived with ten of the Mire. They were met with a party, and the Doctor had Einarr activate electric eels, which tore the Mire's helmets from their heads, revealing their true form. They retreated back to their ship when Ashildr, a Mire helmet atop her head, projected the image of a sea dragon over a harmless wooden puppet. "Odin" was humiliated and driven back to their spaceship, and they left Earth, promising to return. (TV: The Girl Who Died)

Appearance
Outside their armour, the Mire resembled large, clunky, metal robots with guns fixed in their arms. Inside their armour, they have small heads with a few circular rows of teeth.