- You may be looking for the Mire Beast.
Whilst wearing their armour, the Mire resembled large, clunky, metal robots. Underneath their helmets, they had small, lamprey-like heads with three circular rows of teeth. They fed on adrenaline and testosterone; to acquire these substances, they would capture and liquefy individuals from other warrior races, and drink the resulting "nectar".
Although they were known across the galaxy as fierce and powerful warriors, the Mire were, in reality, an unskilled and inept army. They constructed their reputation by picking their battles carefully, only getting involved in fights with weak opponents to ensure they would always win. They also used their advanced technology to appear more powerful than they truly were, to intimidate their foes into submission. (TV: The Girl Who Died)
The Mire possessed technology significantly more advanced than that of Earth.
Their armour - which they wore even when out of combat - was equipped with wrist-mounted energy weapons, whilst their helmets contained communicators and battlefield medical kits. The helmets could also generate holograms, and transmit those holograms to other Mire soldiers. Though the Mire considered this technology an advantage, their reliance on "viewing reality through technology" was a weakness according to the Doctor.
The Mire's spacecraft was enormous and capable of travelling at great speeds through both air and the vacuum of space. It contained the facilities used by the Mire to liquefy their captives and extract the desired chemicals; it also contained their weapon forges. Though the scale and exact purpose of the forges was uncertain, they were loud enough to create sounds similar to thunder when operated from within a planet's atmosphere. The Mire would operate their forges before going into battle, to incite fear in their opponents.
Though their technology was highly developed, the Mire were unsettled by the sonic sunglasses, which - as noted by Clara Oswald - were "technology from a civilisation vastly more powerful than [their] own". (TV: The Girl Who Died)
The Twelfth Doctor and Clara Oswald encountered the Mire in a 9th century Viking village on Earth, after their leader - disguised as the Norse god Odin, whom the Vikings worshipped - projected a holographic image of himself across the sky. He invited the warriors of the village to dine with him in Valhalla, but this was a trap; in actuality, the Mire transported the warriors to their ship, where they were liquefied for sustenance. Clara and Ashildr were also teleported aboard the ship, but were spared because they possessed the sonic sunglasses; the Mire feared the advanced alien technology the glasses represented.
Though Clara was almost successful in persuading the Mire to leave Earth in peace, Ashildr - distraught after the death of her friends - declared war on the Mire, vowing to "crush [them] on the fields of battle". Their leader, eager to defeat the already crippled village, accepted the challenge and beamed Clara and Ashildr back down to Earth.
When "Odin" arrived in the village with ten Mire footsoldiers the following morning, they found the villagers not preparing for combat, but partying and dancing. Confused, they failed to realise that the celebration was a distraction; the villagers, under the command of the Doctor, had constructed an elaborate trap for the Mire, which involved using electric eels and metal wiring to play havoc with their equipment. The plan culminated in Ashildr using a stolen Mire helmet to project the image of a terrifying sea dragon over a harmless wooden puppet, which the Mire were convinced was real. Outmatched, they fled to their ship, leaving their leader alone.
Afterwards, the Doctor revealed the truth of the deception and also revealed that Clara had recorded the entire humiliating defeat on her mobile phone. On danger of having the footage uploaded to the Galactic Hub, "Odin" and the rest of the Mire fled the planet, promising to one day enact vengeance. (TV: The Girl Who Died)
A Mire battle helmet ended up in UNIT's Black Archive in the early 21st century. (TV: The Zygon Inversion) Similarly, a helmet was also found inside the Matrix on Gallifrey, suggesting that the Mire once tried to infiltrate the Matrix but were captured by the Cloister Wraiths. (TV: Hell Bent)
Behind the scenes
According to DWA15 12, the Mire planned on launching a surprise invasion of Earth in 2016. UNIT cracked the code of their invasion plans and deciphered it, with the help of the reader. The Mire's secret message was, "Let us feast on their warriors".