The Mirror of Night is an important mechanic in Hades. Players learn to use it early on after the first escape attempt. It grants several upgrades that can be leveled up to increase your survivability. These upgrades are earned by purchasing them with Darkness and Chthonic Keys. Here is a complete list of all the available abilities in the game.
There are two lists of abilities available to players in Hades, one list is Red, and the other is Green. These lists share slots on the mirror; you can only use one of the abilities in each slot at a time. Cycle through the abilities by clicking on their name. Whichever abilities are actively listed when you leave the mirror are what you will have on your next run.