The Woe: War of Existences
After the inhabitants of the multiverse realized they were one of many self-recurring universes, they began a quest to find their own Existence Atoms. They developed the technology to automatically scan all atoms being transported, refined, or repurposed. The systems would remove and isolate any atom with a high probability of being their own existence. These suspect atoms were bundled together in stable states and shipped physically to a central identification office.
It took considerable efforts and a degree of time travel tech sharing, but eventually they found a number of their own Existence Atoms. With them they were able to see and make changes to their existence in the past, present, and future. At first they used this to greatly fast track their own technology. They refined and built their past in such a way as to generate a species that could live through Atomic Chain Compression. They started rewriting all instances of their existence with exponentially increasing speeds.
At first most efforts to secure and modify Existence Atoms was done to protect known history. It was realized that the universe's background radiation was their own radiation sent through Existence Atoms during scanning.
Eventually, multiple groups looking to secure their own Existence Atoms started to emerge. As their own technology caught up they rewrote time to fit their own needs. Quickly this got out of hand of any of the involved parties. The Atomic Chains got shifted and modified so much and so frequently that they started to fray. This damage was exponential and completely destroyed all of the Atomic Chains before any attempt to slow or stop it was able to begin.
This act turned the universe from cyclic to one that ends in Radioactive Chain Decay, and brought about the End of Existence. Once people realized the damage that had been done it was too late to patch it back together. The Woe did not end suddenly, or with treaties, but rather as each group fell into society-wide grief and depression.