The idea of a demonic Saiyan transformation is such a cool concept, but as far as the official 'Dragon Ball' canon goes, there isn’t one. The closest we get is maybe Super Saiyan Rosé from Goku Black, which has this eerie, almost otherworldly vibe, but it’s tied to a divine entity rather than demonic forces. Fan theories and fan-made content love exploring this idea though—imagine a Saiyan powered by dark energy, with glowing red eyes and jagged aura spikes. There’s so much potential there!
Toei Animation or Akira Toriyama could totally introduce something like this in future arcs, especially with how the series keeps expanding the power ceiling. Until then, we’ve got stuff like Super Saiyan God’s crimson hair or Broly’s berserk form to scratch that itch for 'dark' transformations. I’d kill to see a demonic Saiyan in action, though—maybe fused with Majin magic or something from the Demon Realm.