I ask myself this question often, and I really never get a clear answer. Was it a tv show? a book? something I read on TVTropes? Something I synthesized from a variety of sources?? I still don't know for sure.
What I do know is that Heroes of Thantopolis was concieved because I wanted to have a "Spiritual Successor" to my fake TV-show project called Homestead Ghosts. The story is about a ghost and a demon having an adventure at my summer camp, and finding out how the titular club of ghosts all got to the camp. After the main story (which I did never finish writing), I imagined a sort of TV-Mini Movie (yes) for the show that involved Iota, one of the main protagonists, getting back to camp by completing a supernatural gauntlet called The Tournament of Souls. I knew that my long-time character Helene was in charge of this gauntlet, and appeared in the story to bring Iota to it.
After that story, I read a poem for my 11th grade English class called "Thanantopoist" or the like, and I thought the name looked like it could be the name of a city. So I made it into Thantopolis, and wrote a story for that same English class called "Journey to Thantopolis", where my character C (who makes an appearance in the actual comic) and another ghost character have an adventure there one night, seeing the sights. That was in March or so of 2011.
So then I come back to a night at Pep Band, October of 2011, when I decided I wanted to do a spin off of Homestead Ghosts about Thantopolis. The original story was more in the vein of its inspiration: primarily a mystery with an overarching myth arc. Then it changed pretty quickly into a take on a traditional hero's journey. Since then, the story has largely remained the same (all the same twists, which I will someday get to share, as its near inception). But it still remains an unusual mish-mash of some hero's journey tropes (there are three main "tests", the story takes place in a "special world") with strong inversions and subversions of others (the Mentor figure is corrupt and a technical antagonist to the hero; the hero is thrust into the journey without a chance to avoid or receive the call)
While the narrative is of obscure origins, some other stories and fictions had influence on it from other standpoints. Homestuck, one of my favorite things at the time I wrote the story, had a huge initial influence that I've toned down a bit (but not too much see: this whole third chapter). More recently, Steven Universe has been a source of artistic and stylistic inspiration. And, most egregiously, a whole story arc is dedicated to Julius Caesar, which, since you guys don't know me that well, I must inform you: I love Julius Caesar, especially the Shakespear play (Ehm - Cyrus's weapon is a reference to that play).
So there you have it. Was the question answered? Well, I got to write a lot anyways. Thanks for reading.