What the Seer told him is that he would not cast the curse and he knew it to be Cora’s offspring and wanted her firstborn who from his PoV was the one to cast it. Now, if he made the mistake of seducing the wrong child to the darkness I would expect that that ties into the “undoing” that the Seer spoke of. Henry being his grandson was a big blind spot and he thought he had cheated that one when everyone got back to Storybrooke. Who’s to say another blind spot in his prophesizing won’t pop up?
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Custodian of Graham's darts, Rumple's spindle and Robin's quiver