Delirium

Delirium is the final boss at the end of The Void chapter. It is added to the Afterbirth † DLC. It is assumed to be one of the true final bosses appearing in final chapters of the game, along with Mega Satan.

Description
Delirium's true self is a gigantic, white skull-resembled entity sporting large eye-sockets consisted with black eyes and yellow iris. Its mouth sport long vein-like "stitches", similar to Hush.

From all forms Delirium takes, each are identical to respective bosses from the previous chapters, except with Delirium's own colors, including white colors, yellow eyes, and a "melting" mouth.

Behavior
Delirium can teleport randomly around the room, and send out wave of white projectiles.

Unlike other bosses, Delirium has a unique ability to shape-shift into all bosses from previous chapters, including both normal, posthumous and alternative types, as well as final bosses. However, Delirium does not shape-shift into forms based on minibosses (such as Seven Deadly Sins, Krampus, etc.), although it still can take forms based on the angels. Delirium can also transform into Lil' Haunt, the only monster it can turn into that is not a boss. All various forms Delirium shifts into are listed in the table.

Trivia

 * So far, this is the only boss to have the most amount of health than any other previous bosses.
 * Delirium's boss transformations are chosen by previous bosses the character have fought in the run. This can be proven by running a mod, going into the console, beating 1 or 2 bosses, then summoning Delirium.
 * Delirium has an unused form of Hush's second form, which can be found within the game's files.
 * Delirium's true appearance could slightly resemble Hush's unused skinless form.
 * The word "delirium" is a serious disturbance in mental abilities that results in confused thinking and reduced awareness of the person's environment.
 * Delirium's name may actually suggest by its shape-shifting ability.