Henry dying, in my opinion. I know some people are saying that he’s the only one keeping Emma there, but maybe if his last words were “Please, believe…they need you…” or maybe if he nearly dies and her magic saves him….
It has to just hit her hard and be painful in order for her to believe.
I agree, it would have to be Henry that convinces her. It would be cool if she accidentally uses her magic, and her hands glow or something and then Henry is healed. Hopefully the use of her own power will be enough.