A fanfic or fanfic series where Sombra is the main/one of the main antagonists without having a single direct line in-story.
Sure, Sombra can speak, as inferred from his subordinates, but we will never witness him say anything “in quotes” in the story. Partially as a gag that nods to his lack of speech in the show, but partially because it would be an interesting contrast to Chrysalis, Tirek, et. al. to have a villain who doesn’t speechify or monologue when he has the upper hand over the heroes, and would rather just try to be genre-savvy and kill anyone who opposes him outright (who comes within spellcasting range).
Since just about any direct encounter with him would most likely prove fatal to somepony, he’s fought mostly indirectly–through mind-control helmets, mentally-broken acolytes, cursed objects that show your worst fear, automated defense systems, massive armies of mind-slaves approaching from the north, etc.