In the deepest recesses of the Abyss, a solitary figure sits atop a throne moulded from failed usurpers.  This figure has no known name, for there is nobody that remains alive who can utter it.  To the Lords and Archdukes of the Abyss, it is simply the First or the Demon King for it preceded all those who carved out dominions in the Lower Planes.   

Aboleths know of the King but even they cannot recall its origins for this entity is even more ancient than them.  Some say it was formed from the first act of evil, born intrinsicately from the fabric of the Abyss itself.  Others say it is a facet of Ao, the overgod sent to oversee the Lower Planes and act as its ultimate goaler.  Whatever its origins, the Demon King is considered the purest form of sin, undiluted compared to lesser generations, its existence concealed to all but the mightiest denizens of the Realms.  

In the King's grasp is the 'World's Heart', a relic stripped from a forgotten deity and whose corpse fuels the fires smouldering on top its back.  The Heart is attuned to the fabric of the Material Plane and with a squeeze, the King can bring natural disasters and drought to unsuspecting kingdoms.  Should the King ever crush the Heart, entire civilizations will crumble and die, bringing the death screams of billions and the destabiliztion of the pantheon as their source of power is snuffed out.

The King is completely alien in appearance for it existed before sentient life emerged from the primordial soup of creation and demonkind evolved to ape the bestiaries of mortal fears.  The First of all Demons has no need to change for it has always been the apex predator, pre-eminient and immutable even as the eons gave rise to mighty empires that eventually crumbled under the weight of time and hubris.  

The King does not speak, it does not bargain nor does it need worshipers.  It sustains its existence simply by syphoning off the negative energies of the Abyss, intruding through every layer regardless of territory claimed by other Demons Lords and deities.  The greater powers do nothing to resist because those that tried are dead or have suffered worse fates.  They fear it more than anything in existence for the King lies beyond their power hierarchy and could end their hard earned influence with a careless sneeze.  Even the Greater Devils know not to include the King in their intrigues lest their millenia long machinations be crushed with a contemptuous shrug of its shoulders.  

In the end, the most terrifying asepct of the King is the certainty it will outlast everyone.  The First is utterly indifferent to mortal affairs or the gods themselves for they are mere specks in the canvas of time.  It will be there after they fade into obscurity just as it will be there for the final heat death of the universe.