Board Thread:Spoilers!/@comment-6175354-20140722174741/@comment-24386552-20140728024235

Or in a tragic note, he died during the time period after the Frozen Movie and before it moves into OUAT, and is mentioned that way. And with new villains being introduced, it would be likely Hans or Elizabeth Mitchell's character killed him. Or one of the Frozen heroes, in order to make a grave sacrifice.