The Lava Wyvern had no reason to reject Rhazor's offer. She needed help, and there was only one person in all of existence who could help someone like her.
The Forbidden Race was forbidden in this universe, so unless there was another Forbidden who didn't live by the rules, then helping her was out of the question.
But Rhazor didn't care about the rules, and he could make others not care about them, at least to some extent.
So, knowing the kind of people the Forbidden Race were and the kind of power they wielded, he chose to help her.
If, after losing her bloodline, innate talent, and all kinds of things, she was still able to hold him back for that long, then making her a subordinate was only right.
Later, when and if he got the chance to go back to the Forbidden Universe, or at least to the chunks that remained of the once-powerful universe, having her by his side would ease the hearts of the forbidden beasts.
