The Eyes of Hades greatly resembled a purple amulet. A means of transportation, it was capable of opening rifts in space through which spacecraft could travel. It emitted exoplanetary energy, on such a scale that it could be detected from across the galaxy. It operated by taking advantage of death; as the Doctor surmised, "Every single death is a tiny fracture in reality, and the amulet can lever the fracture open. Primitive, but effective." (TV: The Woman Who Lived)
Before or during the year 1651, Leandro - the advance scout for a Leonian invasion - crash-landed on Earth; the Eyes of Hades, which had been onboard his ship, was lost in the impact. Leandro tricked the immortal human Ashildr into aiding him with its recovery, claiming to be the last of his kind.
Somehow, the Eyes found its way into the possession of Lucie Fanshawe, who mistook it to be simply a beautiful piece of jewellery. Ashildr, posing as her highwayman alter-ego 'The Knightmare', first attempted to steal the Eyes from Lucie's carriage whilst she was travelling with her husband; this attempt was thwarted, albeit accidentally, by the Twelfth Doctor. Later, he assisted Ashildr in breaking into the Fanshawe's estate, and she retrieved the Eyes there.
Following the recovery of the Eyes, Leandro tricked Ashildr into activating it by using the criminal Sam Swift's death as an energy source. Leandro's true plan was revealed as the Leonian invasion fleet began to actualise through the rift. However, the attack was thwarted when Ashildr saved Swift's life using a Mire repair kit modified by the Doctor, causing the portal to close. (TV: The Woman Who Lived)