The Decreator went by a number of names, such as the Antigod, the Unmaker, the Eye of Horus to the Egyptians, and the Eye of Shiva to the Hindus of India. It is the equal, opposite, and shadow of God created when God uttered "Let there be light!", making it the first shadow and the first instance of darkness in the world. It initially appeared as a giant, bloodshot eye in the sky. When it appears it makes the sky around break into pieces and the stars surrounding it blink out. It starts decreating slowly, first words, then books, then shelves, then libraries, etc. It was believed to be unstoppable and it possibly is, but it is possible to slow it down long enough for no one to notice what was going on. An entity of powers born out of the true, ultimate darkness, was born at the very same moment, and it came to be known as the "Great Evil Beast" or the "Ultimate Darkness".
The Cult of the Unwritten Book believe that when God created the Earth he said 'Let there be Light!' and those words cast a shadow which became the Decreator - an entity of pure annihilation unable to be reasoned or even stopped. However when the entity managed to manifest in the mortal plane with the aid of said cult members of Doom Patrol succeeded in slowing him down to such point that no one would notice the slow disappearance of the universe by using similar vibration equal to his powers.
As the "Shadow of God", the Decreator has no personality, no moral and holds no ties to good or evil, simply decreating things not out of hate or pleasure, but because it's in its nature.