[TOMT] [MOVIE] Old Western cartoon, Medieval Fantasy Setting. 3 Characters set off on some sort of quest. 20th Century

I watched this movie on VHS a few times when I was 4 or 5. I forgot it as soon as I was in school, been trying to find it since. So these two characters accompanied by a follower in this medieval fantasy setting are trying to obtain this thing, don’t know what it is but it’s supposed to fix the problems of I think this kingdom they live in, volunteering to set off themselves. Typical fallacy stuff. One scene that I remember the most is this scene where the princess of the kingdom that they are trying to save starts singing and it transcends into a musical kind of scene, only for her to slowly transform into a dragon while singing and become a villain of the movie in the process. I’m unsure if she got given a potion or did it out of her own free will (Somehow) but that’s a scene I remember well. Other than that the story is a blur. I remember nothing else. The animation looked like it came out in the 80s or 90s, I don’t think it was Disney.