The Pitch-Black Stone is an unlockable feature in Hades 2 through an Incantation. You can interact with this item to challenge various Trials bestowed by Chaos. The Pitch-Black Stone appears in the training grounds and will not be available without first encountering the Primordial Originator. There are currently nine Chaos Trials available in the early access of Hades 2 and each must be unlocked before they can be cleared.