Although dialogue-heavy (by Merlin's standards) The Disir turned out to be an enthralling and pivotal episode, compounding the dark and mature tone the show has taken as it has grown over the years.
The story primarily focussed on young Mordred's first official patrol with the knights of Camelot, hunting down a murderous sorcerer called Osgar (Andrew Tiernan).
Only when the knights find him he doesn't attack Arthur but instead presents him with a rune-marked token of The Disir, the supreme seers of The Old Religion.
This disc is their judgement on Arthur, which at first he dismisses as superstitious nonsense, until Merlin and Gaius convince him to take it seriously.
Meanwhile Merlin has been consulting The Great Dragon about his concern that Mordred is fated to be the death of Arthur and The Dragon advises Merlin that he should kill Mordred at the first opportunity!
Arthur takes his trusted knights to pay a visit to this trio of seers, but even then - although Merlin warns them that the place is sacred - Arthur and company behave arrogantly and flaunt their pig-headedness.
The result is a magical scuffle that leaves Mordred mortally wounded - having thrown himself into the path of an attack on Arthur - and once Arthur gets him back to Camelot Gaius admits that the young knight's injuries (being magical in nature) are beyond his abilities to cure. The only hope to save Mordred, he explains, is The Disir themselves.
So Arthur and Merlin return to the sacred cave of The Disir, with the king now much more humble, only to be told that Arthur must embrace the Old Religion - and welcome magic back into Camelot - if he wants his knight healed.
It's here that Merlin makes a surprising proclamation that will have massive repercussions for his own situation, especially given the surprise twist in the tale that follows.
The Disir wasn't exactly an all-action episode, but more about world-building and establishing a mood and layered backstory for the show.
As with all the episodes so far this season lots of places got name-checked, many that had been mentioned in passing in earlier seasons, which all goes to help build up the Truth of the story that it takes place in a 'real' mythical land.
The show is doing a good job of taking what has gone before and building on it.