This theory came to my mind after I read some of death rattles that still have no meaning
Turns out there are some sentence that foreshadow a great body of water that fall from a very long storms. That storms can possibly be everstorm/highstorm or perhaps combination of both
A man stood on a cliffside and watched his homeland fall into dust. The waters surged beneath, so far beneath. And he heard a child crying. They were his own tears —Collected on Tanatesev 1171, 30 seconds pre-death, by the Silent Gatherers. Subject was a cobbler of some renown.
I believe this rattle refers to szeth’s feeling. Sinovar, Szeth’s home, have many mountain that surround it. Will sinovar crumble then got flooded?
I'm cold. Mother, I'm cold. Mother? Why can I still hear the rain? Will it stop? —Collected on Vevishes 1172, 32 seconds pre-death, by the Silent Gatherers. Subject was a lighteyed female child, approximately six years old.
This one refers to a rosharan people feeling amidst this very long storm.
Light grows so distant. The storm never stops. I am broken, and all around me have died. I weep for the end of all things. He has won. Oh, he has beaten us. —Collected on Palahakev 1173, 16 seconds pre-death, by the Silent Gatherers. Subject was a Thaylen sailor.
This might implied that it will be cause by odium (Now Todium). Very long storm will cause the sun to be blocked so sky will turn into night. You might also can find ‘long night’ foreshadowing from this :
So the night will reign, for the choice of honor is life... —Observed circa Ishi 1173 by Taravangian. Subject was King Valam of Jah Keved.
The day was ours, but they took it. Stormfather! You cannot have it. The day is ours. They come, rasping, and the lights fail. Oh, Stormfather! —Collected circa Tanat 1173 by Taravangian. Subject was a young boy.
The darkness becomes a palace. Let it rule! Let it rule! —Collected on Kakevah 1173, 22 seconds pre-death, by the Silent Gatherers. Subject was a darkeyed Selay man of unknown profession.
I believe this will happened in book 5. Brandon need a bitter ending for book 5 so stormlight archive can continue through book 6 to 10. I believe Todium will somehow create eternal global storm on roshar. This will cause catastrophic apocalyptic event that make living people of roshar migrate to shadesmar.
Remember that shadesmar have reverse ecology to the real world? Land in shadesmar is sea in real world while sea in real world is land. So this flood will make shadesmar more occupiable with more land than before. But I believe not all of roshar will be flooded. Mountain will still stand tall above waters so urithru and horneater peak most likely still be safe.
This will become a great strategic tool to make dalinar lose. If dalinar lose, Todium will have him to become his general to conquer the rest of cosmere. With this, Todium will be not afraid to lose. If dalinar win, he will still need to have conquer another cosmere’s world or at least will have to find any new world to occupy. They can’t live in Shadesmar forever. After all, if they can, why did people of ashyn (Human in Roshar ancestors) conquer roshar in the first place?