Leonian

A Leonian was a lion-like humanoid race who once attempted to invade Earth using the Eyes of Hades, an amulet-like device delivered to Earth by their advance agent, Leandro.

History
Leandro crash-landed on Earth and lost his amulet 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 eventually succeeded in tricking her into activating the amulet by using Sam Swift as an energy source, at which point his lies were exposed as the Leonians began to invade. However, their attack was thwarted when Ashildr saved Swift's life using a Mire medical kit modified by the Twelfth Doctor, causing the portal to close. The final act of the Leonians on this occasion was to execute Leandro as punishment for the failure of their invasion. (TV: The Woman Who Lived)

Culture
Not much is known about Leonian culture, but what we have seen and heard from Leonardo is that they exist in tribes and punish failure by death. They are also shown to be very aggressive attacking defenseless towns people. They also seem to use technology that causes death in their enemies such as the Eye of Hades, whether they simply do not know how to make the technology that doesn't cause death or chose not to find another solution is unknown.

Biology
Leonians look like lion human hybrids, they have impressive strength, claws, and lion like faces. They can also breath fire.