The final arc of the Rurouni Kenshin manga is indeed about a man, Enishi, who has an old grudge against Kenshin. However, the final arc of the manga was never put into the anime. There do exist two OVA series that adapt this part of the anime.
The Trust and Betrayal OVAs adapts the backstory of the final arc, and the Reflection OVAs adapts the final arc, though it only appears as a lengthy flashback. The main story of Reflection takes place after the manga.
Both sets of OVAs are drastically different in style and tone from the manga and anime, and lots of content from the manga is not present due to the brevity of the OVAs. That being said, they were highly praised. They were released in English under the Samurai X name.