Leonian

The Leonians were a lion-esque humanoid race. They once attempted to invade Earth using the Eyes of Hades, an amulet-like device capable of opening rifts in space, delivered to the planet by their advance agent, Leandro.

Biology and culture
Leonians were humanoids that resembled lions. They showcased impressive strength, and possessed powerful claws. Their eyes could glow a fierce red, an ability they seemed to be able to switch on and off at will. They could also breathe fire.

They lived in tribes, and punished failure by death. Leandro, one of their own, was shown to be very aggressive, attacking defenceless townspeople. The Leonians made use of technology that manipulated and induced death, such as the Eyes of Hades. (TV: The Woman Who Lived)

History
Leandro crash-landed to Earth and lost the Eyes of Hades - and thus his key to opening the portal that would enable his brethren to invade the planet - in the year 1651. He forged an unlikely alliance with the immortal human Ashildr, claiming that the rest of his race had been wiped out; he falsely stated that he wished to use the amulet to escape Earth, and offered to take her with him. He eventually succeeded in tricking her into activating the amulet by using the highwayman Sam Swift as an energy source; the Leonian fleet then began to actualise in their plane of reality through the rift, exposing Leandro's lies. However, their attack was thwarted when Ashildr saved Swift's life using a Mire repair kit modified by the Twelfth Doctor, causing the portal to close. The final act of the Leonians on this occasion was to remotely execute Leandro as punishment for the failure of their invasion. (TV: The Woman Who Lived)