It's an insane miracle that the show as renewed in the first place due to it's ridiculous ratings, especially this past Season.
In the 4th episode of the 6th Season, we unravel the Strange Case of Dr. Jekyll and Mr. Hyde. Long ago, Jekyll cared deeply for a woman named Mary, her Father was head of an Academy that Jekyll desperately wanted his work to presented at. Unfortunately, Jekyll's hope is denied and he is soon visited by Rumplestiltzskin who offers him a deal. Jekyll drinks a serum that turns him into Mr. Hyde in hopes of getting the job he wants and of course Mary. That all backfires when Jekyll awakes in Mary's bed and with the serum poisoning his mind, he begs Mary to love him and accidentally pushes her out a window. Back in present day Storybrooke, Mr. Hyde and The Evil Queen have teamed up and have an encounter with Mr. Gold, only for him to learn that Hyde cannot be killed, not even by the Dark One's dagger. To protect Belle and his unborn child from harm, Rumple traps Belle on Hook's ship, eventually to learn that you can't kill Hyde unless you kill Jekyll. This all results to Regina talking to Emma and realizing, if she wants to kill the Evil Queen, she has to die too.