What actually happened with Adam's father(s) at the end of Good Omens?

Question

Towards the very end of Pratchett and Gaiman's Good Omens,

after Adam has decided not to destroy the world after all, and the moment of the destined battle has passed without incident,

his father, the Devil himself, is on his way up to the surface:

The ground began to shake. The noise was like a subway train, but not one passing under. It was more like the sound of one coming up.

Crowley fumbled madly with the gear shift.

“That's not Beelzebub!” he shouted, above the noise of the wind. “That's Him. His Father! This isn't Armageddon, this is personal. Start, you bloody thing!”

Aziraphale, Crowley, and Shadwell march forth to face Him, but Adam does ... something:

Adam looked around. He looked down. His face took on an expression of calculated innocence.

There was a moment of conflict.

But Adam was on his own ground.

Always, and ultimately, on his own ground.

He moved one hand around in a blurred half circle.

... Aziraphale and Crowley felt the world change.

There was no noise. There were no cracks. There was just that where there had been the beginnings of a volcano of Satanic power, there was just clearing smoke, and a car drawing slowly to a halt, its engine loud in the evening hush.

Apart from an absolutely cringemaking overuse of the word "there" in a single sentence and paragraph ... what actually happened here? Adam's biological father, the Devil, is coming up from Hell, then

suddenly it's only his adoptive father, Mr Young. Did Adam somehow transform the Devil into a human? That seems unlikely. Did Adam dismiss the Devil, and it's only coincidence that Mr Young turned up exactly there and then? I think there's something I'm missing.

Answer 1

The answer is in something Crowley said earlier.

He actually mentioned this regarding young Warlock, who at the time, he still thought was the Antichrist.

"Nobody's going to notice anything out of the ordinary. It's reality, angel. And young Warlock can do what he wants to that, whether he knows it or not."

Adam has, of course, been unconciously rewriting reality to hilarious effect for most of the book. Probably the most obvious demonstration is this:

Where the reactor should have been was an empty space. You could have had quite a nice game of squash in it.

Right at the bottom, all alone in the center of the bright cold floor, was a lemon drop.

Outside in the cavernous turbine hall the machines roared on.

The power plant is still running, you'll note. Cause and effect officially took a flying leap off a cliff. This is what he's capable of without actually intending to do anything.

So Adam faced a reality where his father (full-on flaming Satan) was coming to have a stern word with him. Except... now he has full awareness of what he can do. He shut down the Apocalypse without even relying on that, just with the right words and friends. And well... he's standing in reality, isn't he?

But Adam was on his own ground. Always, and ultimately, on his own ground.

So he rewrites reality... to one where his father was coming to have a stern word with him.

The symmetry might even have appealed to him, even if he wouldn't actually know it. Aziraphale and Crowley have a fair bit of chat about his sense of humor in other minor changes like Aziraphale's bookshop being restored. I imagine Satan was terribly surprised at no longer being about to erupt onto the Earth... and that probably also had the intended message to not try it again. But he's not that important to the story in the long run.

Answer 2

It's not in the book canon, but in the serie just released in Amazon Prime Video.

When Satan arrive on earth, Adam said to Lucifer :

Adam You're not my dad. Dads don't wait until you're 11 to say hello, and then turn up to tell you off. (...) If I'm in trouble with my dad, then it won't be you. It's going to be the dad who was there.

Lucifer What did you say ?

Adam You're not my dad. You never were.

Then Adam proceed to "change" Lucifer into his real dad. As Neil Gaiman is involved in the creation of the book and the serie, we could say that the serie clarify this part of the book (that I didn't read).