WARNING: The following contains spoilers for Star Wars: Darth Vader #12, by Greg Pak, Guiu Vilanova, Dean White, Giada Marchisio and Joe Caramanga, on sale now.
Anakin Skywalker's tragic turn to the Dark Side is one of the most devastating stories ever told in the Star Wars universe. In the Prequel Trilogy, Yoda heavily speculated that fear would lead Anakin away from the light, but it appears there's something beyond the fear that helped push Anakin to the Dark Side. As Star Wars: Darth Vader #12 shows, pain is equally important in fully driving Vader's power.
After Vader nearly betrayed Palpatine in Star Wars: Darth Vader #6, Vader had been put through countless trials. These trials reached their pinnacle when Vader attempted to confront Palpatine on Exegol. Vader's armor was beginning to fail after he defeated a giant space creature and Palpatine's army of cultists. Once Palpatine was assured that Vader would not revolt against his control, he brought his apprentice to be repaired on Coruscant.
As the repair droid began to work on Vader, he announced that he was going to power Vader down. Vader objects and instead elects to be conscious during the invasive procedure. The Sith Lord enters into a force vision as a result of the pain, recalling his removal of Luke Skywalker's hand. He ponders if the pain he brought to Luke will lead his son towards hate and power. This is a play on Vader's perspective of how he came to the Dark Side.
Yoda famously said that fear is the path to the Dark Side as fear leads to anger which leads to hate and eventually suffering. While Anakin was afraid of losing his mother and Padmé, he was ultimately afraid of the pain it would cause him. Anakin fully realized this pain when his mother died in Attack of the Clones.
Vader's belief that pain is what leads to the Dark Side is further defined in his battle with Obi-Wan in Revenge of the Sith. By the point that Obi-Wan and Anakin faced off on Mustafar, Anakin had already turned away from the Jedi. Even though Anakin had felt the fear of loss, most of his actions had not followed the predictable natures of one's descent towards the dark. It was not until Kenobi sliced through Skywalker and critically injured him that Anakin fully embraced his hate. The pain was the final pin that led Anakin to become Vader.
When Vader came out of his visions in Star Wars: Darth Vader #12, he had a renewed focus. It's clear that the pain he felt during his repairs allowed him to channel the Dark Side on a new level. Where Vader has recently been depicted as unsure of his path, he awoke with clear determination. The pain allowed him to become more in touch with the Dark Side and to grow stronger.
Vader is more or less an android at this point. While some of his biological organs remain, he is much more droid than man. Through these operations to keep him alive and at the peak of his strength, Vader has surely felt a tremendous amount of pain. This could very well explain Vader's immense strength and why he appeared to be more powerful in the ways of the Dark Side than many of the other Sith.