https://www.gamespot.com/articles/assassins-creed-origins-update-adding-nightmare-di/1100-6455374/

A big new update for Assassin's Creed: Origins is coming this month. In addition to adding a Final Fantasy Chocobo horse (yes, really), the update will add a new difficulty level called "Nightmare." As you could guess, this is going to make the game ultra-difficult. The official patch notes explain that enemies will be "more resilient to damages and more dangerous." There is no word, however, about what else is changed.

The December Origins update also adds a much-requested feature: enemy scaling. This means the game will automatically scale enemies of a lower level to your current level. This is an optional feature, so you don't have to have it turned on. Additionally, the update expands the Heka Chest offerings by adding new Nightmare and First Civ items.