It

A nameless, shapeless creature of pure evil, which originates from the Abyss.

In the Abyss
It is generally considered to be the most powerful denizen of the Abyss. Even the Demon King trod with care near this being. In the Abyss, It occupies the deepest portion, and all who fall into It's dark web are tormented and destroyed. Wherever It moves to, all others flee, except for the Grotesque, which held it's ground at it's temple though terrified. It is actually capable of opening portals between the Abyss and our world, and may have been responsible for the creation of the Grotesque's temple in the first place. It has accumulated a number of familiars which it sends through the temple, including the Beast.

In our World
It is regarded with fear by the human race. If the creature ever had a name, it was purged and forgotten out of fear of invocation. It is known to be malevolent, purely and unceasingly. However, when in our world It is immaterial. It can assume the forms of inanimate objects, and a focused individual could discern his presence by observing blurred distortions radiating from the object like heat waves. During periods when the Abyss is opened, It will emerge and roam. Doc's group was responsible for shutting down gates that would open at seemingly random intervals. They knew the Demon King was their enemy, but failed to comprehend all this slaughter was done for the amusement of a far worse evil. Darren was the one who eventually discovered that the Thrakr demon universe was leaking into the halfway universe and led a team to shut down that pathway. This portal had been open for centuries. It draws strength from suffering, and wandered the gateway universe periodically to harvest, especially during the cultist period, until Norman Phillips destroyed them and brought peace.