There seems to be a misunderstanding in your question. Obi-Wan Kenobi did not turn to the dark side after the Clone Wars. In the Star Wars saga, Obi-Wan remains a steadfast Jedi throughout his life. After the Clone Wars, he goes into hiding on Tatooine to watch over Luke Skywalker and continues to serve the light side of the Force.
It is Anakin Skywalker—Obi-Wan's former apprentice—who turns to the dark side and becomes Darth Vader. Obi-Wan's role is quite the opposite; he is a key figure in preserving the Jedi teachings and protecting the next generation.
If you are referring to a specific piece of fan fiction, alternate universe storytelling, or another media where this occurs, please clarify!