Sentinels of the Multiverse: OblivAeon
The heroes of Earth and beyond have fought against the effects of an unknown cosmic power for a long time, and each time they managed to prevail. However, other realities and timelines have not been so fortunate. The goal of this cosmic entity's encroachment upon the Multiverse has been the collapse of all realities and timelines into a singularity which will self-implode, leaving naught but OblivAeon, the true heir to nothingness.
OblivAeon itself has manifested on Earth in this timeline to put an end to the pitiful resistance. If it succeeds, everything will be destroyed. However, if the heroes are able to stop OblivAeon, the cracks which run through all realities will be repaired as the cosmic power that created them is destroyed, effectively cutting the timelines and realities off from each other.
One way or another, the Multiverse ends here.