In the final moments of his fight with his former apprentice Darth Vader on the Death Star, Obi-Wan smiled as he saw that Luke Skywalker was watching the battle, sending a worried chill through what remained of Vader’s body that he was being tricked.
After striking down his old Master, Obi-Wan disappeared into the force, leaving Darth Vader in a shocked confusion. Despite this, a feeling of immense joy began flowing through Vader and the Dark Side began swelling, but this quickly changed as the final moments of the fight began racing through Vader’s mind once again, panicking the mechanical man.
The Dark Lord soon started believing that this was another Jedi trick, and that Obi-Wan must not have taught him everything he knew. This left Vader terrified that Obi-Wan would return to ambush him, and finish the job that he couldn’t on Mustafar.