Well, I've been working on a fanfiction called "Ancient Equestria." It was supposed to cover several historical events. The Sombra event does appear but it's scheduled for later in the story in one of the much later chapters. Does it then count as a Sombra fiction, or is it just not enough to be relevant since he only appears [or is set to appear] in one chapter?

I think it can count as long as he's a key part of the plot, even if we don't really get to know him. (like in the actual episode he was in! We know nothing about him, but he was still important, so it's the Sombra episode!)

Ah, okay. Well he certainly is very important to his chapter. He's one of the main characters which is supposed to explain who he is and why he was dark.