An episode of Star Wars: The Clone Wars season 3 has Anakin Skywalker turning to the dark side a year before becoming the Sith Lord Darth Vader in Star Wars: Episode III – Revenge of the Sith, and shows just why Mustafar was so important to his transformation. While Anakin becomes a Jedi again by the end of the episode, the circumstances of his brief fall from grace speak to the reasons why he’d ultimately become a Sith a year later.
The Clone Wars season 3, episode 17, "Ghosts of Mortis", is the final part of the Mortis trilogy, where Anakin, Ahsoka, and Obi-Wan find themselves in a mystical plane that exists on no star maps and is inhabited by three god-like Force-wielders. Each Force-wielder represents a different aspect of the Force. The Father appears to be the cosmic Force, The Daughter seems to represent the living Force, and The Son embodies the dark side of the Force. Hoping to destroy The Son, Anakin ventures into his domain, the Well of the Dark Side, where The Son assaults him with visions of his future as the Sith Lord Darth Vader. In the previous episode, The Son mesmerizes Ahsoka into briefly falling to the dark side, but unlike his padawan, Anakin turns entirely willingly.
The visions that The Son shows Anakin are the catalyst for his turn to the dark side. Horrified by what he saw, Anakin was desperate to prevent the premonitions from becoming reality at any cost. For this, he made a Faustian bargain with The Son, embracing the power of the dark side of the Force. The Son’s machinations are thwarted when The Father removes Anakin’s memories of the visions, reversing his turn to the dark side. Anakin’s dilemma on Mortis is not unlike the circumstances seen in Revenge of the Sith., right down to taking place in a sinister, magma-filled chasm with a striking visual similarity to Mustafar. Anakin is desperate to prevent visions of his wife dying in childbirth from becoming a reality, and the Jedi offer no solutions to his problem, so he turns to the Sith in desperation, believing Palpatine’s promise that the power of the dark side would save Padme.
To Anakin, the power of the dark side and the Sith are simply means to an end. Anakin feels held back by the Jedi Order and the patience required to master the Force, making him particularly susceptible to the philosophy of the Sith and the addictive qualities of the dark side. The tragic irony of Anakin’s turn is that his desperation to prevent his visions is what ultimately leads them to become reality. By the time that Anakin is left on the shores of Mustafar, his body destroyed, his wife dying, and the galaxy doomed to autocracy, he has nothing left but to serve the Sith and Galactic Empire as Darth Vader.
The Well of the Dark Side’s strong resemblance to the planet Mustafar was obviously a deliberate choice on Star Wars: The Clone Wars’ part to have Anakin’s corruption on Mortis resemble his fall from grace in the prequel trilogy, but the similarity makes sense in-universe as well. In the canon continuity, Mustafar is itself corrupted by the dark side and has a long history of association with the Sith. The Well of the Dark Side was the source of the dark side’s power on Mortis, so it’s fittingly similar to a planet steeped in the dark side.