Board Thread:Spoilers!/@comment-25361971-20150928100931/@comment-3010035-20150928111706

My Theory is, that any time the Dark One's curse is ended, then the Darkness escapes.

So in Skin Deep, if Belle had completed True Love's kiss, Rumple would be freed but the Darkness would be at large.

If Rumple had died in the Miller's Daughter, then the Darkness would have escaped and been at large.

Only by killing onesself with the Dagger, can the cycle end, but the Darkness has a fail-safe: The Vault.