How to Reset the Mirror of Night in Hades
Depending on how much Darkness you've farmed from your Hades runs, you may want to reset the Mirror of Darkness to spend gems on different Talents. Perhaps you'd like to have another level of Death Defiance or more supply for your Cast. Either way, you may need to start over and reallocate to get the upper hand.
To reset the Mirror of Night in Hades, go to the mirror in Zagreus' bed-chamber, and interact with it. Scroll down to the very bottom on the right side. You'll see a white looped-arrow icon next to a Chthontic Key symbol and the number 1.
Click this icon/button to spend 1 Chthontic Key to "reclaim any Darkness spent on Talents," as the game says. Once you do so, you're free to reallocate Darkness as you see fit.
Outside of losing a single Key, there's no other penalty for resetting your Talents and effectively performing a character respec. A Mirror of Night reset might just be what you need to get past a particularly pesky section or boss, such as Theseus in Elysium. For more on Hades, such as how to get Ambrosia, how to heal, or how to activate Dark Thirst, consider heading over to our other guides on Supergiant's roguelike.