The oldest known version I've read is probably the one by Charles Perrault from 1697, which is the 'Cendrillon' most Disney drew from, but it's already a softened take. Perrault added the glass slipper and the fairy godmother; earlier variants were far less genteel. In the Brothers Grimm version, 'Aschenputtel', there's no fairy—the magic comes from a hazel tree planted on her mother's grave, and the birds help her. The stepsisters mutilate their own feet to fit the slipper, and in the end, doves peck their eyes out as punishment. It's a harsh moral landscape about cruelty and divine retribution, wrapped in that classic folk tale brutality.
Perrault's ending is all about forgiveness and social elevation. Cinderella not only marries the prince but arranges marriages for her stepsisters with other lords at court. That always struck me as wildly unrealistic but fascinating—a kind of forced grace that's almost more about her own virtue than their redemption. The Grimm ending feels more like cosmic justice, violent and final. The original 'plot', if you trace it back to things like the Chinese 'Ye Xian' or the Egyptian Rhodopis, usually revolves around a persecuted orphan, animal helpers, a lost shoe, and a royal recognition scene. The 'happily ever after' is consistent, but the path there is littered with blood and ashes in many cultures.