The Adamantine Sickle of Kronos is a weapon created by Gaia for Kronos to kill Oranos and rule the world. It appears several times in the game's story as the Sickle of Kronos and plays a key role in Act II and Act III of the main story.
With a bit of luck you can loot it from the telkine Ormenos on Normal, Epic difficulty, probably on Legendary difficulty as well. He wields it as a weapon during his boss fight at the end of the main story chapter Under Wusao Mountain.
The sickle is almost impossible to equip with those requirements, but doing so grants the Steam achievement Sick lewt.