We all want to believe we are heroes of our own story - unless we want to be the villain, of course.
At the end of everything, the Final Reaper decided he hadn’t been a hero. Driven by a desire to right the wrongs he was subjected to, he killed everyone who wronged him or his people - which turned out to be everyone that wasn’t killed by someone else first. He'd won - but it was a hollow victory.
Eventually, Order’s Voice found a way out. If the only existing being would agree to give up most of his power, the Voice could reset the multiverse to an earlier time with a few minor changes. Of course, the Voice couldn't ask it that way. It could only ask if the Final Reaper was willing to start over from when Earth was first brought into Order.
It was an easy decision, and yet it wasn’t. Was he willing to go through eons of pain again to not be alone?
Yes. In a heartbeat.
Not that his heart beat anymore. Now it would. Perhaps he could even be a hero, this time.
When he landed in his old body - more or less - on Earth, the Final Reaper once again became Thomas. He was both and neither. He needed a new name for a new life.
While this is technically a System Apocalypse story, it's a System Apocalypse that is designed to have a large percentage of the population survive and prosper. There are a lot of problems that come with the appearance of the Voice, and it's entirely possible to lose. Earth has some special opportunities, but also special challenges.
The first time around, Earth won the first round and lost the second. Serenity has ten years from when the Voice arrives to prevent that from happening. It will be a group effort; Serenity can't win alone - which is difficult for someone who's been alone for as long as he has.
Of course, that's only his second priority.