Short answer: I can't remember.
Longer answer: I wrote the first draft of what was then called 'Set around 2008 (and possibly the year before) and after roughly a thousand edits and rewrites, it was published in early 2013 before going out of print a year later. Not that long ago, I know, but I've written a lot more since then and before reading through the original book last year, I hadn't done more than skim it in in the time since. My tale of angels and demons and a threat to all of reality felt like it had been written by someone else other than a few lines I knew were all me. I still liked it, though, which is where the new title and whole doing it myself thing came in. But for where it started ten or more years ago...
I think I had a vague idea for a short story about a guy with a terminally ill daughter who gets involved in a battle between mythical creatures. Events would take place over the course of a single night and he would gain some way of treating his child. Pretty vague and not that original, I'm sure you'll agree, but a concept I was fond of as I've always liked putting the supernatural right beside the familiar. At the time, I wrote a lot more fantasy than horror although my fantasies were becoming quite a bit darker as I went on. 'Set obviously took that basic idea and changed it to a mother who's lost her child. Ditto a change to the other characters, the timescale and the stakes. Instead of a one night fight between whatever creatures they were going to be, I ended up with a joint effort between Heaven and Hell to stop a blockage in death which will eventually lead to the potential destruction of everything.
Structurally, one of the biggest influences was the TV series 24 with Jack Bauer blowing the shit out of the bad guys. While nobody in my book demands to know who anyone is working for or for Chloe to open up a socket, damn it, the growing scale of events and threat 24 was so good at came to mind often while I was writing. That's why the my threat keeps getting worse. Also because it's fun to really fuck things up for your characters, as well.
Ultimately, the book that began as 'Set and became Dead Sun has been a long time in the telling and while I can't remember where the basic idea came from, maybe that's not as important as the tale finally being told.
Dead Sun is free on Kindle Unlimited over here.