Abyss is a boss in Red Hood’s Woods you can get to right after defeating the Mad King and progressing further into the Abyss. In the Abyss, you have to light the pillars and then the Abyss will initiate a fight with you.
Defeating the Abyss will give you the Dark Side Moon skill, and teleport you back into the Basement, allowing you to go through the final mirror.
