This is a chapter about an oath.
We don't swear many oaths in our day to day lives. I don't imagine they were ever especially common in any time period. People don't make solemn oaths while buying groceries, whether in a modern grocery store, or a medieval village marketplace. But in the 12th century oaths were avoided because they held so much weight. Now they are avoided because they have little to no weight at all. Whether you're a witness at trial, or the president being inaugurated, if you break your oath it will only matter if you get caught and can't explain away your actions. In other words, they are only a legal matter.
But what if an oath is a spiritual matter? Jesus told us not to make oaths at all. Our yes should be yes and our no should be no. But we keep using them all the same, because we want them to be more than a "yes" or a "no". We want them to hold some kind of magic that will bind our souls to the promise.
Sentrus certainly believes that. He believes it so devoutly that he is uncertain how to make the vow the Queen calls on him to make in this chapter. In the end, he leaves out the most important part of any vow: the object on which it is made. He leaves it out, because the Queen asks him to make the vow upon "whatever he holds most dear" and he cannot answer her with what that dearest thing is. And he can't make something up. If he makes up a dearest thing in the world, or says the wrong thing then the oath might have not value. Or worse, he could have made a vow on something he could not identify, which means, if he broke the vow then he has no way of knowing what it is he loves that will be destroyed.
In order to make an oath, you have to search yourself to your furthest depths. First, you have to decide if you are capable of fulfilling your vow. Then you have to decide whether the thing you are swearing on has the value you think it has. Finally, you have to decide if you can live with the consequences of failure. In this chapter, Sentrus will find himself without answers to any of these questions. And only by getting to the end of the story will we, the readers, find out.