The Idea of Evil is an eldritch god from the fictional universe of the Berserk manga and anime.
The Idea of Evil is an eldritch entity, the God of the Abyss. Which came into existence because of humanity's need for transcendental reasons for what happens in their lives. Reasons for life and death, reasons for why their lives are filled with sadness, pain, and suffering. The entity acts on the collective desires of mankind and is therefore responsible for their fate.
It's a many-eyed flesh-like heart that lays at the core of the Abyss. Which is a realm at the deepest level of the Astral World. It's known as the "Void" or "Hell" to some, though no one is tormented or punished within the real. Souls are purged of their individuality and form an interconnected chain, becoming lost within the realm's space and matter. Those whom make deals with demons or have made sacrifices to the God Hand, end up in the Abyss.
The God Hand
Serving the Idea of Evil are the God Hand, the most powerful of the demons. Who execute the will of the Idea of Evil, with their own desires belonging to the entity. The God Hand have manipulated human history by orchestrating various important events. Each of them was once a human, whom during their mortal life was the owner of the Crimson Beherit (the Egg of the King), an arcane artifact which gives you the world in exchange for your blood and flesh. It appears every 216 years, and the owners are destined to throw away their humanity so they could be reborn as angels of the God Hand. The God Hand are also the masters of the Apostles. Each of them is able to alter reality (Void & Femto - spatial, Conrad & Slan - physical, Ubik - metaphysical). Though having incomparable foresight and being cognizant of the law of cause and effect, none are fully-fledged gods. Their forms were molded out of the collective negativity of humanity, and their essence exists wherever their are major concentrations of human thought.
The Apostles are humans who summoned the God Hand, and in exchange for sacrificing something precious to them, they are turned into demons. The sacrifice is something that defines their humanity, and therefore when they give it up, it's as if they gave up part of their soul. There is only one absolute law that the Apostles follow, which is "Do as thou wilt", which permits them to use their ascended being as they see fit. Though they can do as they please, most of them will answer the call of the God Hand and those whom are loyal to the God Hand will kill those whom interfere in their designs.
No two apostle have a similar true form, which is the reflection of their soul made flesh. The forms vary widely, from warped versions of animals to hellish and demonic abominations. In turn their human features often in some shape or form their true form. Most of the apostles have an insatiable need for bloodshed and lack empathy, though some are able to suppress these tendencies. All of the apostles have a humanoid form, but at any moment can turn into their true form. In their human form, they possess superhuman reflexes and strength. When in their true form, they display incredible powers, ones beyond what most humans possess (e.g. amorphous body that cannot be harmed or supersonic flight). Some of the apostles can also create pseudo-apostles, the method depending on the Apostle.