If you've been looking for a way to increase the difficulty in your Hades 2 runs or if you're hoping to get more Nightmares to upgrade your weapons, unlocking the Oath of the Unseen is exactly what you're looking for. The Oath of the Unseen provides players a way to add challenging modifiers like increased enemy health or disabling Arcana cards to each run. In exchange, if Melinoe defeats the designated boss with these challenges enabled, she'll be rewarded with a Nightmare.