so after they enter the mushroom kingdom mario and luigi greets them only to get interrupted by bowser jr as shadow mario (he got control of the paintbrush again) after defeating them bowser reveals stuff such as he (disguised as one of her minions) encouraged meleficaint to leave her world and take over radiant garden as well as free pete. he also caused xenahort to become obsessed with the X-blade and therefore cause all the other games as he knew that he could easily take over all the worlds if his plans were stopped somehow. he also wanted to rule all the worlds after he discovered kingdom hearts and abandoned aiming for just the mushroom kingdom. he decided that he needed a way to get the last 8 worlds "softened up" by planning to get riku to go to the remaining uncharted worlds as the mushroom kingdom was already softened up by mario and luigi.then they fight bowser as his normal form. then the artificial KH he tried summoning appeared and he turned into terabowser (since terabytes come after gigabytes and he is more powerful than giga bowser was in the smash bros games.) he then kills mario and luigi and disables commands (except items such as a potion or an elixir) they fight him and win then the KH shatters, bowser dies, and mario and luigi come back to life.