Mire

The Mire were one of the deadliest and most advanced warrior races in the galaxy.

History
They were fed by stamina, strength, adrenaline 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 their 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 Twelfth 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)

A Mire battle helmet ended up in UNIT's Black Archive. (TV: The Zygon Inversion)similarly there was also a Mire battle helmet inside the Matrix on Gallifrey which means the Mire once tried to infiltrate the Matrix but were prevented from doing so by the Cloister Wraith (TV: Hell Bent)

Appearance
Outside their armour, the Mire resembled large, clunky, metal robots with guns fixed in their arms. Inside their armour, they had small, lamprey-like heads with three circular rows of teeth. (TV: The Girl Who Died)

Technology
They were provived with a medical kit who could make a human immortal. (TV: The Girl Who Died et al.)