Searching for a 90s Anime with a Dark-to-Light Princess Twist

Question

When I was about 5 years old, I watched a show that I've often thought about and tried to remember, but I've never been able to identify it. Please bear with my ambiguous and fleeting memories.

The show was either an anime or a cartoon, but I recall the style being more similar to anime. I saw it in the 1990s, but my country tended to air older shows, so it might have come out earlier than that.

All I remember is that there was a black castle with a dark princess whose main goal was to spread darkness throughout the land. She received directives from a voice behind a stained glass window/mirror.

The heroes were trying to stop her, and they were also searching for a light princess, who they believed could save the land.

The only other thing I remember is that, in the end, it was revealed that the dark princess was actually the light princess who had been corrupted. When she reverted to her old self, the darkness lifted from the castle and the land.

Answer 1

This could be "Peter Pan: The Animated Series", a Japanese anime originally aired in 1989 and subsequently dubbed and broadcast in many other nations.
While the first episodes are (more or less) an adaptation of James Barrie's book, the series then introduces original characters and story lines.

In particular, what you remember could be the final story arc (episodes 24 to 41) where Sinistra the witch, Queen of Darkness, wants to extend her control over all Neverland using the power of her Black Mirror.

Her granddaughter Luna is destined to become the new Queen of Darkness, but she escapes and spends some time with Wendy, Peter and the Lost Boys, telling them only the White Mirror could break the magic of the Black Mirror.
Luna is eventually recaptured by Sinistra and brainwashed, so that she thinks Peter Pan is her enemy. In the end Wendy reaches her and help her fight her inner darkness, and the Black Mirror turns into the White Mirror, restoring the ruined Neverland.