Celestia is uncertain about one of her previous students: Marjoram. The unicorn is highly intelligent, incredibly powerful in magic, and yet he has no friends. He works as one of the few morticians in Canterlot, dabbles with herbalism, and is more than okay with performing autopsies on all sorts of creatures if it means furthering his research.
On the other hoof, Celestia has Twilight Sparkle, new to being a student. Young Twilight is skilled for her age, and recently discovered her special talent in magic. She has no friends outside of her family, and this worries Celestia; for she's seen what can happen to good ponies who feel isolated from everyone else.
So, Celestia decides to meddle in fate, and tasks Marjoram to give Twilight at least one lesson per week. Chaos, of course, can only ensue.
Edit. Nov 29th/2020: As some of you might have been able to realize, I haven't updated this story in a long time. I wrote myself into a whole and couldn't dig myself out of it. However, over the course of quarantine, I've been rewriting the story with the intent of completing it, even if it takes me a few years. As it stands, I'm a little over 16k words into the rewrite, and once I hit 20k, I'm going to publish and start posting semi-regularly.
In celebration of hitting 100k words, I comissioned the talented Pridark for a commission of Marjoram.Pridark