Merlin's hard work
The feature of this story that makes it stand out from other Arthurian tales is that the story depicts Merlin hard at work for the future king. Although the novel later claims that "God gave the sword to Arthur," that's only really half of the story. Technically "God" gave the sword to Merlin, after a lot of hard investigative work that finally led him to the right place. Then he had to defend the sword and serve as steward of the holy chamber where it was kept. He is always strategizing and plotting so that things will go according to plan, then at the last second, he hides all his work and let's everything happen as if it were by God's own doing.
The Macsen sword
The sword is an ironic symbol for Merlin because he doesn't use it as a king, but instead he guards it. That is an ironic relationship to have to a weapon, because as Arthur later shows, the sword is a tool of defense, but Merlin is the defender of it. Also, it is ironic as a symbol because the sword is cast in many lights throughout the story in various dreams and portents. Merlin knows that the sword is not a perfect solution to every problem, but rather, it can be a perfect tool for Arthur to execute his noble justice.
Combat training
There comes a beautiful moment in the plot where Merlin has the honor of teaching Arthur and a friend to defend themselves with swords. The training is doubly ironic: It is ironic because typically the story of Arthur begins with him as an excellent warrior, so this is a use of dramatic irony, a peak behind the veil so to speak, like Yoda and Luke Skywalker. Also, it is ironic because Merlin is teaching Arthur how to vanquish foes but not in the name of death and murder, but as a godly king who can defend the weak.
The hero in the wilderness
Arthur is an ironic character in the story. Although he is the center of Merlin's world, he is not the center of the novel. Merlin is constantly sacrificing for him to make his path successful, as a parent who lays down their life for their child. Merlin is the silent hero in the novel, because he guides Arthur through a developmental process in the wilderness. Arthur is a hero, but in training. He is dependent on Merlin's guidance, but eventually, the goal is for Arthur to be perfectly independent and responsible.
The magic coronation
The scene where Arthur becomes crowned as king occurs in a church building. Merlin invites the crowd to an ironic game. He sets the sword on fire and makes it white hot to the touch. Then, instead of saying, "Arthur is the true king," he invites the community to be king. Whoever can lift the sword shall be king. This is an ironic way of showing the community that Arthur truly is the best candidate because they all have a chance to be courageous, but they defer to him.
