After seeing Anakin start to kill Padme Amidala with a Force choke, Obi-Wan, having snuck aboard the Naboo senator’s ship, realized that his apprentice was too far gone, and one lightsaber duel later, Ob-Wan came out victorious by finding the high ground.
He believed Anakin would die from his injuries and massive burning, but because Emperor Palpatine arrived in the nick of time, Anakin was brought back to Coruscant and turned into the cybernetic Darth Vader we love and fear… more the latter.
In case it’s been a while since you last watched Star Wars Episode III: Revenge of the Sith, Anakin Skywalker and Obi-Wan Kenobi locked lightsaber blades on Mustafar after nearly all the Jedi in the galaxy has been killed by Order 66 and Anakin slaughtered the Separatist leadership on the lava planet.