Perhaps Cora tells Regina that if they have the dagger, they can control Rumple, which in turn means the three of them have a combined power that the Charmings can’t stand against, which means they’ll have to give Regina what she wants and hand Henry over.
But Cora’s actual plan is to get the dagger so she can take Rumple down, kill him, and steal his powers for herself.
Also, I’m still entertaining the idea that Regina is onto her, at least so far as knowing that she’s not genuinely helping her get Henry back, so she’s playing along to lull Cora into a false sense of security, until the moment when she can take her out once and for all. I’d have a whole new respect for Regina if she pulled a stunt like that.
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