When Anakin Skywalker transformed into the Sith Lord Darth Vader in Star Wars: Episode III - Revenge of the Sith, Anakin was slowly consumed by hate and anger. As a result, he became more aggressive with a lightsaber, much to the disapproval of his former master, Obi-Wan Kenobi. There was no shame in how much he enjoyed fighting, but Anakin always felt his potential was hindered by Obi-Wan. When he pledged himself to the dark side and became Vader, there was no holding back.
After Vader's lightsaber duel with Obi-Wan on Mustafar, Vader's body was left mutilated, and he was confined to an armored life- suit. These restrictions forced him to change his lightsaber fighting style. Like Anakin, Darth Vader used Form V, also called Djem So, but because his mechanical suit severely limited his physical movements, he had to adapt and learn two other lightsaber disciplines. The first was Form III (Soresu), which was more defensively-minded and non-aggressive, and the second was Form II (Makashi), which prioritized precise and efficient movements while avoiding disarmament. Vader even studied a third form, Dun Möch, and with all three styles, he was nearly unstoppable.
Djem So Was Perfect For The Added Weight Of Darth Vader's Attacks
For much of the Clone Wars, Anakin used Form V in battle. Darth Vader also used lightsaber Form V, and the idea behind this technique was to end a fight before it could be allowed to begin. It was a heavy-handed style and executed with aggressive, powerful strikes. The created their own opportunities to attack while their opponent was left helplessly waiting for the perfect moment to strike. Djem So was a perfect fit for Anakin due to its violent nature, but more so for Darth Vader because of the added physical strength his cybernetic limbs offered.
Because Anakin pledged himself to the Sith and was no longer fighting for the Jedi Order, Djem So became another way for Obi-Wan Kenobi "Part III," in which Vader overpowered, and proved his dominance over, the weakened Jedi.
Darth Vader Learned Soresu To Compensate For His Weaknesses
Being too offensive-minded would have been the Sith Lord's downfall. For example, in the Obi-Wan Kenobi finale, the second duel between Obi-Wan and Vader resulted in the control on the Sith Lord's chest module being heavily damaged. Obi-Wan wounded Darth Vader with a painful saber slash to his back soon after and Vader even had his iconic mask cut in half.
Because of these deadly blows, which could've been avoided, Vader studied Form III (Soresu) to improve his defensive tactics in battle. Leaning Soresu also became crucial to his survival as Vader needed to protect his life- system to stay alive. It created a formidable hybrid lightsaber style. With great skill in both offense and defense, Darth Vader became even more dangerous in Star Wars than he already was.
