Ok, this might not be the thread for crackpot theories, but Merlin’s background story is nagging at me, so I’ll toss it out here. @The Watcher I think nailed it — the whole lost love seems overly simplistic.
If Merlin’s lost love = original DO (or first DO tied to the Dagger) = Nimue, and if we are indeed doing the whole Underworld thing for the second half of the season, is it possible that DO magic originates from the Underworld — like a bit that got trapped on the wrong side of a realm, and either mutated into or was harnessed as dark magic? Perhaps because the original DO (Merlin’s lost love, or her immediate predecessor) wanted more and more power, and caused some sort of rift between realms meant to be kept separate?
Also, is the Darkness actually sentient? Not just like a vaguely self-preserving parasite, Alien(s) style, but an intentional being?