The Dark One is a boss found in the Caves and in the Catacombs.
- It roars then sends out a ring of 12 projectiles
- If Isaac moves below it, it fires a brimstone laser downwards
- If Isaac moves to one of its sides, it dashes twice across the screen, veering slowly towards Isaac while charging.
- It dims the room which makes its other attacks harder to see.
Upon entering the room, if the player goes toward the bottom-center of the screen, and The Dark One uses his Dashing attack, there is a chance that The Dark One will freeze on his Dashing frame, and remain in the same state for the remainder of the fight.
- This bug has been observed when entering from the left and right sides of the screen.
- The Adversary is an advanced version of The Dark One.