Ao is the overgods of Toril and the lord of all the deities. Only the gods (and the old Elminster) know of his presence because he doesn't answer to the preyers of mortals.

How to draw an overgod? Someone who is everything and nothing at the same time? I don't know, but I made this draw and I like it. So this will be Ao, or at least one of its form.

"...It was then that the gods understood that their fate, whatever it would be, had been sealed long before this summoning. That terrible silence filled the great hall, but it was soon shattered.
"Keepers of the Balance, I address you one and all!"
It was Ao's voice, and in that voice was heard the power of a being so great that the gods fell to their knees in response. Lord Bane alone managed to place only one knee on the pantheon's cold floor.
"Most noble was your heritage! Yours was the power to stave off the ever-present threat of imbalance between Law and Chaos, and yet you chose to act like children, resorting to petty thievery in your quest for power..."
Bane suddenly wondered if the being who had given the gods life long ago had called his creations to this place to undo his mistake and begin anew.
"Extinction may be your future yet, Bane," Ao proclaimed, as if the Black Lord's thoughts had been spoken aloud. "But do not let it concern you, for that fate would be most merciful compared to what shall soon befall you — and the other gods that betrayed my trust."
"No longer will you ignore the very purpose for which you were given life! You shall know your transgressions and remember them for all time. You have sinned against your liege and you will he punished."
Bane felt the coils of darkness approach.
"The thief!" Mystra shouted. "Let us discover the identity of the thief for you and return the tablets!"
Tyr, God of Justice, raised his arms imploringly. "Let us not pay in kind for the foolishness of but one of our brethren, Lord Ao!" Darkness, like the lash of a whip, slashed across Tyr's face, and he fell back, screaming and clutching at his now useless eyes.
"You see nothing but the salvation of your own skins!"
The gods were silent, and the dark bands darted between them, drawing the gods closer to each other, as if herding them together to create a single target for Ao's wrath. The gods cried out — some in fear, some in pain. They were not accustomed to such treatment.
"Cowards. The theft of the tablets was the final affront. You will return them to me. But first, you will pay the price for a millennium of disappointment."
Bane stood his ground against the bands of energy, and suddenly the biting strands of darkness erupted into blinding flames of cold blue light that seared him. He turned from the light and caught a glimpse of Mystra as she, too, held her ground, a slight smile etched across her features. Then the bands caught Bane, and his world became pain such as only a god could imagine or endure.
After an eternity of torment, all the gods were caught in the dark bands of power and drawn tightly together. Only then did the deities find movement and thought once again possible.
And fear. This they knew intimately...
Suddenly the pantheon seemed to vanish and the gods, still bound together, found themselves staring full into the face of what frightened each the most — chaos, pain, love, life, ignorance. And each god saw his or her own destruction there, as well.
"That was but a taste of my anger. Now drink deep from the goblet of a true god's rage!"
A sound was heard then unlike any other.
All the gods, save Helm, were cast from the heavens.
"You will take an avatar." Ao's voice resounded in their minds. "You will possess the body of mortals and live as humans. Then perhaps you will appreciate what you once took for granted."