The Fall: Or Dodge in Hell is a sequel to the much better REAMDE. It also firmly establishes REAMDE as taking place in the Cryptonomicon universe, something that was not previously apparent. Enoch Root is one of the characters who ties the whole thing together, as is the Waterhouse Foundation.
It's hard to spoil the whole plot of the novel, mostly because there isn't one. But I will reveal the mystery of Enoch Root. Here we go:
The Cryptonomicon universe is a simulation. Enoch Root was transmitted from the universe in which Cryptonomicon is embedded into the Cryptonomicon universe.
Relevant quotes:
On p. 80 we first find out this is a sequel to Cryptonomicon, not just REAMDE:
Waterhouse Brain Sciences Institute. I took the liberty of getting in touch with one of them, Ben Compton, whom I have stayed friends with.” “Is this the Waterhouse from the weird cyber bank?” Alice asked. “That Waterhouse?”
On p. 244 we Enoch Root enters the story. I'm not going to spoil this one because it happens in one of the only two actually good sections of the book. (The other is Moab.)
On p. 699 the Societas Eruditorum returns, or at least is mentioned:
Calling a meeting of the Societas Eruditorum?” C-plus asked.
“You’re the only member left,” Enoch said. “You’re a one-person meeting.”
On p. 927, we finally get the answer. Enoch tells Zula that their world is a simulation. He comes from the world that's simulating the world of Cryptonomicon, though he doesn't fully recall his existence in the previous world or his purpose for being in this one. To provide some context, at this point about two thirds of the previous 900 pages have been spent building and living in a simulated universe where the dead are resurrected.
“Makes you want to go there, doesn’t it?” asked a man’s voice, just next to her.
She turned to see that older chap who had made the joke earlier, and recognized him as Enoch Root.
“Did you really just ask me if I want to die?” she shot back.
He just got a wry look and said nothing. As if she had caught him out in some mischief.
“Why don’t you have a go,” she suggested, “and send me a message back from the next world?”
I still have responsibilities in the previous one,” he answered.
“So this has happened before?” she asked. She began strolling over toward the visualization of Landform 2, where Dodge—Egdod—the REAP—whatever you wanted to call him—was poised, wings spread, above the top of his dark tower.
“Perhaps not this,” said Enoch, walking by her side, “but—”
“You know what I mean.”
“Yes,” he confirmed.
On p. 931, we discover that Enoch's purpose was to keep the chain of simulations going, to make sure that the simulated world of Cryptonomicon births a simulation of its own:
This is where I leave you,” said the voice of Enoch.
“Where on earth are you going?” Zula asked, looking about for him. But she had lost him in the radiant fog. His voice was clear, though. It was the only sound in the world.
“I’m not entirely sure,” he admitted. “But I’m done here. I did what I was sent to do.”
The golden light grew until it was all she could see, and his voice was heard no more.
It's a not a bad ending for the character, or a bad revelation. In the two decades since Cryptonomicon was published, I don't think anyone saw this coming, and yet it makes sense and is a reasonably satisfying answer. I just wish The Fall hadn't been such a truly terrible book in most other respects.