Hi i am big fan of h2o i want to ask what happen to Rikki in season 3 at the episode "Dark side" i don't understand

I am a big fan too. I think what happens is since everybody is hiding from the tentacle, when she try's to destroy it listens to her. She has control over it. But the other putri duyung don't.

I want to know to I didn't really understand it that much either please help