The Adversary

The Adversary is a boss that can appear:

Behavior[]

Quickly fires Brimstone laser towards Isaac if he is seen in any cardinal direction. The beam bends slightly; following Isaac's movement
Flies off-screen into the air and plummets down on Isaac's position a few seconds later, creating a circular rock wave on impact which expands to 4 rings.
Summons 3 Spiders.
Dims the room, causing the Curse of Darkness effect for a period of time.

The Brimstone attack's homing effect is stronger as it gets farther away from The Adversary. It is much easier to avoid by staying close and being ready to pivot around him via quarter-circle dodges when he telegraphs the attack.
If there is a room containing two Adversaries, their Brimstone shots may be unavoidable if fired from both sides of the room simultaneously.
Host Hat and Pyromaniac make Isaac immune to The Adversary's rock wave attack since it is considered to be the same as explosions by the game.
3DS If The Adversary dims the room, pause the game and the dim will eventually go away while still in the pause screen.

The Adversary in-game.
The Adversary as one of Delirium's forms before Repentance.
Early concept art for the Adversary on the bottom right corner, along with other Rebirth boss concept arts.

He is the posthumous version of Dark One.
"The Adversary" is one of the ways that the original Hebrew word "ha-satan" can be translated.
"Adversary" is the name of a level in Super Meat Boy, which Edmund, part of Team Meat at the time, worked on.
The Adversary appears to be a severely injured Dark One, though what caused such injuries is unknown.
The Adversary has a bandage on his horn in-game, but he does not on his boss portrait.
The Adversary is one of the three bosses that can darken the room (a temporary Curse of Darkness effect). The other bosses are Dark One and The Lamb.