Sundream is Cor Caroli's Astral form.
Annoyed by his initial defeat, Sundream Cor Caroli enters by hurling a hypermassive star at you, destroying an absolutely massive amount of the area (before this theme starts). You manage to somehow avoid being vaporized, but have been swept away to a different area entirely. You have 2 choices. Stay and defeat him (which would warrant some kind of reward), or run (you would be taken back through brief sections of previous stages, eventually making your escape. Cor Caroli will not pursue you outside of the astral realm for his own reasons).