Spoiler!
This is their third timeline. The fragments gaeul? (I don't remember her name) saw with her powers are from the previous timelines. In the first one, after the west sea rift broke out, humanity failed, J sacrificed himself and the artisan Hong ye-song (he was a very smart guy in the first timeline) created an artifact using the remnants of Hunter J and information gathered before humanity's collapse. The artifact basically capable of preserving and transmitting information across a reset of the world.. Which just ended up overlapping the first timeline with another one instead of rewinding time. and LSY used that artifact.
That information became the foundation for the second timeline, where humanity was far more prepared and managed to avoid many of the disasters that destroyed the first. However, the second timeline also ended in failure, leading to yet another reset.
The story's present is the third timeline. The visions and fragments Gaeul sees through her ability are not predictions but actual memories from those previous timelines bleeding through the cracks of reality. Eui jae himself is an anomaly carried over from the first timeline through the West Sea Rift, which acts as the focal point connecting the resets. That's why certain people experience déjà vu or inexplicable familiarity. The current world is essentially built on the accumulated sacrifices that came before it.
As for LSY, after repeated failures and resets, his self from the previous timelines becomes deeply tied to the mechanism that keeps the Rift from fully breaking open. And rather than simply fighting it, he had to serve as an anchor against the Rift's expansion which traps him inside the rift ALL ALONE (my baby) and kinda going insane even after the timelines reset. That version of LSY appears later in the novel and he tried to take over the current LSY’s body. That version of LSY was very depressed, and showed an unhealthy possessiveness towards J because imagine being trapped in the West Sea Rift and witnessing the same apocalypse loop over and over again, where every timeline ends with J’s death or disappearance and there is never a version where he gets to keep him. (It was a sad arc, I cried my eyes out)
Anyways, that's basically it. Lmk if you want further spoilers
I rmb reading that the humanity collapse was humans mutilating into monsters & it will rain white powder (iirc), and when J in 3rd timeline enter the west sea rift again there was this part about synchronisation error that one I didn't really understand the implications
So in the first timeline LSY become the west sea rift?
No, TL1 LSY died early because he never awakened. When TL1 J used the watch, it effectively turned back time and erased the remnants of TL1's world.
The problem is TL2. TL2's world did NOT get erased properly because the watch wasn't supposed to be used a second time, and TL2 LSY was not enough to be an anchor (which made him be a warden that's keeping TL2's world from merging with TL3's world, but that didn't last long, that's why the apocalypse started happening in TL3. In a way, the West Sea Rift is the remnants of TL2's world)
When TL3 J entered the West Sea Rift, the rift's entrance disappeared because TL2 LSY wasn't enough, so it sought out J (because he was the original anchor and is powerful enough, i.e. TL1 J using the watch). I didn’t really get the synchronization part right before J got spat out of the rift but I'm assuming it's because he knew too much (TL2 LSY was there apparently) so the system probably wanted to erase J's memories

i gave up after ch 309...it's too heavy of a read for me. anyone mind spoiling what each timeline is and how it's connected and how it linked to that rift?