The conclusion, I think, keeps this otherwise strong two-parter out of the feature range.
The Merlin Wiki has a plot summary here.
I will say this: If The Darkest Hour is a herald of things to come, then I think we're about to experience a real - and welcome - shift in tone. The cinematography is different, the opening credit sequence is different, and - probably most importantly of all - Bradley James and Colin Morgan are changing how they portray their characters. Arthur and Merlin are not boys anymore; as the aforementioned new credit sequence notes, they are now young men. After a year of ruling Camelot as Prince Regent - a year during which Uther's health, both mental and physical, has steadily declined - Arthur now carries himself with less bravado and more gravitas -- and Merlin has matured just as much.
There's also the relative darkness of the story to consider. That Morgana is willing to sacrifice her ailing sister to unleash the Dorocha upon the world is a testament to how far she has fallen -- and the Dorocha are, by themselves, a legitimately frightening enemy. We still ride the family-friendly line here, but you might want to put your very small children to bed before watching these episodes. Occasionally, the Dorocha even made me jump, and I'm an adult!
I should emphasize, though, that the writers still manage to squeeze some humor into the proceedings. The knights frequently made me smile -- especially Gwaine, who agitates a beehive and accidentally sets his socks on fire. (LOL! You are awesome, Gwaine.) Merlin and Arthur, meanwhile, continue their amusing banter ("Good servants are hard to come by." "I'm not that good." "... True."), though the affection that exists between them is becoming more and more obvious. Notice, for example, that after Merlin is attacked by one of the Dorocha, Arthur very nearly ends his quest to the Isle of the Blessed and rides Merlin back to Camelot himself. Awww. They wub each other, don't they?
Meanwhile, back in Camelot, Agravaine is evil. It was kind of hard not to notice that at the very start, so that's a bit of a flaw in the writing there. On the other hand, Gwen is becoming more and more queenly in her bearing, which is nice -- though, of course, she ends up making an enemy of Agravaine because of her confidence.
On the whole, I really like these episodes. As I suggested in my opening comment, though, the death of Lancelot doesn't really have much of an impact -- at least for me. The truth is, I don't think Lancelot is a very deep character. I really haven't seen the man nearly enough to become emotionally attached.
'Tis generally a compelling story, but I shrugged at the "shocker ending."
As I stated above, I like how Morgan and James are growing into their roles.
I also like watching so many characters put themselves aside in order to serve higher goals.