Well, Merlin will always keep his promises. Eventually.
See, Morgause is back, and she resurrected some legendary undead knights. Seven of ’em. They can’t be killed. The only time they stopped before is when the sorceress who gave them power died. Arthur, Merlin, and a bunch of canon fodder knights ride off to take a look, and that’s when Morgause approaches Morgana and starts something.
Wait, why is Morgana still in Camelot? She disowned Uther and let that rebel go.
There’s always been something of an undercurrent to season two that Merlin’s own actions are what makes the Dragon’s various prophecies come true. Sure, Uther’s general anti-magic polices don’t help, like, at all. But we saw Merlin’s attempts to help have made a few enemies and what happens to Morgana in this episode sure doesn’t help. Merlin doesn’t want to do what he ends up doing. He looks for every possible way to avoid what he ends up doing. He still thinks of Morgana as a friend of sorts. Sure, I don’t think he’s ever had much of a conversation with her, but still.
The point is, when Arthur and Merlin manage to escape the undead knights, they return to Camelot to find everyone there asleep. Well, anyone human. There’re a few horses that seem wide awake. Plus Morgana. And, when he finally has to go for help, the Dragon is awake, and this time, he won’t offer any help unless Merlin lets him out. Merlin has to swear on his own mother’s life. That’s binding enough.
The Dragon advises killing Morgana.
Merlin really doesn’t want to do that.
He tries really hard not to.
Even when he sees the knights attack everyone except Morgana.
Even when he offers Morgana several opportunities to admit she knows anything.
Even after he doesn’t tell Arthur that Morgana has magic and she knows this.
So, he uses some hemlock from Gaius’s supply to poison Morgana.
And that means Morgause will bargain to save her sister, and that almost certainly means that even though Morgana is revived, she isn’t going to be Merlin’s friend anymore. And even though the kingdom is awake again, Merlin does end the episode by letting the Dragon go, even though he doesn’t get a guarantee the beast will not just attack Camelot once it’s free.
Look, this show ends every episode with a “next time” and it shows Camelot in flames. We know that stuff is coming.
Besides, the next episode is the finale for series two.