Mission to Mustafar/Legends

Vader's mission to kill all the leaders of the C.I.S. while this part of the mission was successful Vader's old master Obi-Wan Kenobi was there to challenge the young Sith lord the duel raged on until finally Vader fell into lava trying to make a leap toward Obi-Wan this incident was what forced Vader to wear a respirator and full body suit for the rest of his life.