That doesn't excuse using "on accident" in a fantasy novel. This isn't about clinging to grammar rules for their own sake. It's about tone and immersion. "On accident" is jarring, modern, and very region-specific. It sounds like something a kid from suburban America would say, not something that belongs in a carefully built fantasy world.
You're writing for a global audience, many of whom find that phrase not just unusual but grating. "By accident" is neutral, familiar, and doesn't pull readers out of the story. "On accident" does, - repeatedly. Language may evolve, but good writing is also about knowing what not to include if it breaks the reader’s flow.
You're writing for a global audience, many of whom find that phrase not just unusual but grating. "By accident" is neutral, familiar, and doesn't pull readers out of the story. "On accident" does, - repeatedly. Language may evolve, but good writing is also about knowing what not to include if it breaks the reader’s flow.