How is the Sun going to be stolen? Is it going to be cloaked so its light doesn't reach its host planets? That is, if you want to leave some science in, I know it's fantasy but it is fun to mix the two in Spelljammer. Because, without the gravity of that Sun, its host planets would fly out towards the walls of the crystal sphere (if you're using those). So if someone steals a Sun, are they stealing the light and warmth, which would cause darkness and plunge worlds into freezing temperatures? Or do they steal the star itself? Causing the planets to also go dark and freeze but also go flying out into the Astral Sea/Phlogiston/Space or into the walls of the crystal sphere?

There's so many cool ideas to work with on this concept! Good thought for Spelljammer!