Rogue One: A Star Wars Story introduced us to a number of new planets within the Star Wars universe, but one planet was perposely left unidentified - the lava based planet where we are first introduced to Darth Vader. The reasoning for this planet's lack of identity on-screen was likely to keep fans guessing as to which planet it was and also possibly becase it will be explored later on in Episode 8 or 9.
The planet is, as many of you probably guessed, Mustafar - the same planet where Revenge of the Sith took place during the film's climactic battle. Also, the planet where Anakin Skywalker essentially completes his transformation to becoming Darth Vader, following his duel with Obi-Wan Kenobi. This is also the setting for that mysterious bacta tank scene we witnessed in the Rogue One trailers. Unfortunately the hooded figure kneeling in that scene was severely anticlimactic compared to the numerous fan theories the cut-scene generated prior to Rogue One's theatrical release.
YouTuber Stupendous Wave recently uploaded another well done and extremely detailed video outlining why Vader chose to reside on Mustafar and who the hooded figure in the bacta tank scene actually is. The video also covers a number of other Darth Vader related questions many fans had after seeing Rogue One for themselves. Check out the video below and let us know what you think in the comments section below!