Yusri's Bones

Long ago, Yusri, or the creature that we know as Yusri, was a leviathan of sorts, swimming through space. But it is dead now. And somehow, our ancestors found its corpse. This is our home now. We spend most of our time on the surface. But Yusri’s corpse isn’t… normal. It rolls and folds over itself, effectively swallowing anything on it whenever it does. But this doesn’t kill us. On Yusri’s scale, anything in those folds of flesh is crushed. But for us, there are hundreds of feet between the folds. Entire cities can get swallowed into Yusri’s corpse, appearing on a different side of the body months later. Often, their populations have been completely wiped out by starvation, and thus many cities keep massive food stores for this very case.
The bones of this world are despicable things. They do not form any kind of structure, as far as we can tell, and serve no purpose. They float through the flesh of our world, bleeding marrow, leaking blood. And yet even in such places, life finds a way. Or, at least, it once did. For in the dark hollows of Yursi’s bones rest ancient cities made of rusted iron. They are built into the bone, each city, though thousands of miles apart, is built similarly. Each one is composed of a lower city encircling a massive citadel with spearlike spires rising from its rooftops. Massive spiked chains are strung between the towers, and cruel hooks dangle from the links.
But the worst part is the bodies.
The cities reek of death, emanating from the hundreds of bodies. Some impaled upon the spires, others swinging from the hooks overhead. From a distance they appear human, but up close, that can be proven false. They are massive creatures, perhaps 12 feet tall. Their faces are blank. No eyes. No mouth. No ears. No hair. A small ridge where a nose would be, but no nostrils. They wear no clothes, but their crotches are smooth. Their skin is a pale, icy blue. They are inhuman, and yet they resonate with those who have seen them in some unspeakable way. They do not bleed from their wounds, and though they stink of decay they do not seem to have decomposed.
I have seen them with my own eyes. Four weeks ago, the city in which I lived was swallowed into the planet. We collided with a bone as it too floated aimlessly through Yusri’s flesh. And from our vantage, we could see into a crack in the bone and could perceive the city within. I was part of the expedition that ventured into the crack, hoping to find shelter or food. We reached the city and found it empty. Not just of life, but of anything. Save for the pools of blood and piles of marrow, there wasn’t a single thing in the city. No furniture, no tools, nothing. Just iron buildings.
I write this in my city, smashed against the bone. Our only hope for survival is for the bone to breach the surface within three weeks. Else, we shall starve.
How did it come to this?