Satan

Satan is the boss of, and the final boss of Ending 12.

Behavior
The battle against Satan has three distinct phases.

Phase 1
The first phase starts with a slight delay after Isaac enters the room:
 * Satan shouts, and roughly two seconds after Satan's voice dies out, The Fallen spawns at the center, with two Kamikaze Leeches to to either side.

Phase 2
Once The Fallen is dead, the Devil Room statue at the top of the screen transforms into Satan's first form. He stays near the top of the room moving horizontally and performs the following attacks:
 * Shoots half-rings of 8 projectiles from both his hands towards the left and right sides of the room.
 * If Isaac is below him he:
 * Shoots two lasers from his hands, that curve towards each other while leaving a gap in the middle.
 * Charges up and fires a beam from his mouth directly downwards.
 * Spits out 2 consecutive spreads of 5 and 4 projectiles towards Isaac. He chains this attack at least twice.

Phase 3
After depleting his entire second phase health bar and reaching third of his total HP, he flies off-screen and has one attack:
 * Stomps the ground with both of his feet at Isaac's previous positions, similar to Mom's stomp attack but with less delay. The stomps come down one after another and after two stomps they persist a moment before both are retracted.
 * Up to two Kamikaze Leeches continuously spawn during this phase.
 * They can be killed by Satan's stomp but will also deal considerable damage to him.
 * There appears to be a maximum amount of Kamikaze Leeches that can be spawned as they stop appearing if the fight draws out for too long.

Once Satan is defeated, remaining Kamikaze Leeches die alongside him.

Trivia

 * Satan appeared in the Halloween Update for the original The Binding of Isaac.
 * Satan's pose as a statue and during the VS Screen is a nod to the demon Baphomet, who is usually portrayed in a similar pose.
 * The demonic speech Satan shouts at the start of his first phase reuses one of Mom's voice lines, shouting "Isaac!" in reverse with heavy distortion.
 * The music that plays during this battle is titled Hericide. He​ricide is the act of murdering one's own master, lord, or teacher.
 * This music also plays during the battle against Mega Satan.
 * This music also plays during the nightmare wave of Greed Mode, for unlocking the floor's Devil Room or Angel Room.

Satan ‎