Leandro (Leonians)

Claiming to be stranded on Earth after his tribe were overthrown, his world destroyed and his wife killed, Leandro asserts that he is the last of the Leonians. Having escaped through a portal created by an alien artefact in the form of an amulet unique to his homeworld – Leandro found himself trapped in the 17th Century.

Forging an alliance with Ashildr, Leandro promised to take the now cynical and bored ‘Lady Me’ out into the universe in exchange for her help finding the now missing amulet. The presence of the alien artefact in 1651 drew the attention of the Doctor and resulted in his reunion with Ashildr.
The Doctor and Ashildr carry out a daring heist on the Fanshawe house to retrieve the amulet… But when Leandro reveals himself, the Doctor discovers it's dark secret – a death is required to open a doorway to another world! Ashildr has become so disillusioned, that sacrificing a life in order to activate the amulet means nothing to her.

At Tyburn, at the event of Sam Swift’s execution, Ashildr uses the amulet to open a portal and the army of the Leonians is revealed to be alive and waiting to invade. Realising she has been betrayed, Ashildr agrees to help the Doctor and using her second immortality charge, they reverse Sam Swift’s death to close the portal.

Leandro is destroyed by his ‘brothers’ for his failure!

bbc