By a piece of fanfiction. It's Harry Potter, post-Deathly Hallows, and it's so good and so sad and jesus. It made me cry. Let me reinflect that: it made me cry. (There were mitigating circumstances. I was listening to the soundtrack of The Land Before Time while reading it. Still.)
Anyway. It's here, it's called
End of the Line and it's awesome, especially the second chapter.
A passing acquaintance with the Discworld helps, but it's not necessary.