Once you've beaten him, you'll go back to talk to Belle, but will find out that she has disappeared, going after "a man in a black cloak", the same one you'll have seen talking to Beast back in his room. who apparently wants to turn him into a heartless, so that the organization will have control over his Nobody. You'll have to go to the ballroom, the big green door across the main entrance, and fight the boss.
Before that, though, you should take advantage of the monsters in there, and level up to, at minmum, level 15. After, just go inside the ballroom, where you'll find the organization member, Xaldin, but he will left, and you'll have to fight the monster he summons.
This boss is divided in two: the shadow Stalker and the Dark Thorn. To fight the first, the Shadow Stalker, just hit the column and the chandelier when he melts into the, and the use the reaction command "release" to get him out in the open, and hit him. When he melts into the window, run to the place beyond the columns, because he's gonna throw some purple rays which take a lot of life. As for the Dark Thorn, just keep hitting him, even when he is invisible, and try to use the RC to use the chandelier to make hom visible again. But he does have some powerful attaks, so watch out.
After that, just watch the cutscene, when Belle and Beast make up, and then you can go. A new world has appeared, but go first to The Land of Dragons.
You should also go out to the courtyard, through the front door, and open the chests there. There are also heartless, (gargoyles and bats).
Bosses (up to down): The Thresholder, The Dark Thorn, The Shadow Stalker.