Eyes of Hades

The Eyes of Hades was a purple amulet-like device which could open a dimensional portal, but only after it was used to kill someone.

Leandro brought the Eyes to Earth to allow his fellow Leonians access to the planet for invasion. His ship crashed and he lost the Eyes. Meeting Ashildr, he forged an unlikely alliance with her, claiming that the rest of his race had been wiped out. Ashildr eventually recovered the amulet, and Leandro tricked her into activating it by using Sam Swift's death as an energy source. Leandro's true plan was revealed as the Leonians began to invade.

However, the attack was thwarted when Ashildr saved Swift's life using a Mire repair kit modified by the Twelfth Doctor, causing the portal to close. (TV: The Woman Who Lived)