Leto grabbed the book as he was falling down then he was completely submerged in water. Not just any water, this was cold water. Water sitting just at the level where it wouldn't freeze and deep enough to be almost black. Leto looked up and saw the cracked floor he had fallen through, the rays of light pouring down over him. He saw the cracks sealing up after him and the light closing out to leave him trapped in the dark. All around him the water felt nearly crushing against his skin and the cold dug in deep until it felt like it had settled on his very bones. The last line sealed above him and the dark was complete. Leto couldn't see what was around him, how far away a shore was, where the walls of the Keep were, or even his own flailing limbs dragging at the water to try and reach something or somewhere. He was adrift, alone, and very very afraid. But he kept hold of the book. Something deep inside told him that he should, that that oddly heavy tome was important in this endless black abyss.
Soon the obvious problem of air became a real concern. Leto clutched at his throat and kept pawing at the water trying to go higher up. If the floor cracked once maybe he could crack it again, but no matter how much he swam he never seemed to get closer to the ceiling. Pain burned in his chest. Burned was the right word, because it felt hot and rising as if holding his breath for so long would somehow result in his entire chest bursting into fire killing him. The thought was funny for a moment, he might have laughed or given it more consideration if the clawing panic of drowning wasn't quickly consuming his every single thought. When he couldn't hold his breath any longer he started to struggle. Leto's body instinctively began to spasm and twitch as it demanded air that he could not give it. In the end, as it was always going to be, he tried to take a breath. Leto opened his mouth and received nothing. Not just a lack of air which he perfectly expected but no water either. He gasped and gasped but there was nothing to fill his lungs with. His vision might have gone dark but in that all encompassing blackness he couldn't be sure.
What Leto did know was that after a subjective eternity he came to his senses. He was not starving for air anymore. Yet he still felt like he was floating in the waters. He moved his arms and felt the natural drag against them that told him he was still beneath the surface of this dark sea and even more strange that he still had the book clutched impossibly tight in his hand. The satyr opened his mouth to breath in and the moment he tried he once more felt that burning pain of wanting and receiving nothing, but it only lasted a second this time and once he stopped trying to gasp he found himself perfectly fine. Cold and trapped in darkness but not suffocating.
Not breathing at all.
This unsettled Leto greatly but the entire thing was so strange, especially after losing so many in the same day, that he just felt too tired to try and understand the logic of it. Too tired to puzzle it out or reason his way through how he could not need to breath suddenly. He tried to talk but no words came out either, which wasn't that surprising he supposed since he had no breath to speak. Floating with no where to go in the silent darkness, Leto just waited. He didn't know what he was waiting for or what he could possibly hope to see or hear in total blackness. He wondered if this was what it felt like to be dead. He hoped not. He hoped that Adlas and Raona weren't also floating somewhere in pitch darkness.
Time seems odd in the dark. It stretches and twists yet is never exactly as it seems. The dark, true dark, is the natural enemy of a proper sense of time. So Leto had no idea how long it was before he heard it. So soft he might have imagined it except that like always when he heard the sound his long ear twitched. Again and again a twitch of his ear to the faintest of sounds. So faint he couldn't actually make out what it was, like a slow kind of song perhaps or a long droning speech near melodic in its tone. The more Leto listened the more he could slowly start to hear it but it didn't come from one direction but every direction. All around him these sounds slowly built up until they were a constant roaring chorus but as if heard from a great distance away. When there were no other sounds to hear it almost seemed deafening but that was ridiculous as Leto had to strain so very hard to hear it.
Then there were two changes. Two very important changes one following the other.
The first was a series of small twinkling lights. They had not been visible before but Leto couldn't say when they had appeared. They just were and, by some means Leto couldn't explain, had been for a while now. The second change was a subtle shift in the pitch of the dull chorus he was straining to hear. The lights shined for moments at a time and then winked out. Only to return a second later or be replaced by another small light near it. As Leto watched the lights expanded their range and soon seemed so much closer to him even as the chorus grew to the point that he could near feel it vibrating against him. It started with a vague awareness before turning into terrified certainty. These lights were not stars, though they could seem such in the utter blackness around him, but markings that gave off light. Just as the whispering roar he heard, the change in pitch it seemed to occasionally display, was the sound of something truly great in size turning. A dull thunder and the world displaced by its very passing.
Whatever this thing was Leto knew for certain that it was moving toward him and for a terrible moment he got the impression that jaws larger than mountains were closing in all around him.