I'm writing a series of horror novellettes and in order to introduce the main characters, I've decided to write a few short stories. These stories will function as intros, but I don't want to call them that. Thinking in terms of a television show, I've seen this happened, although I can't remember how it's called. What word am I searching for? Is there such a word?
You could call those stories preludes to your main story:
an action or event serving as an introduction to something more important.
"education cannot simply be a prelude to a career"
synonyms: preliminary, overture, opening, preparation, introduction, start, commencement, beginning, lead-in, precursor
"the ceasefire was a prelude to peace negotiations"