One of the noteworthy points about this phrase is that the one verb is used twice, with two different subjects. Mille cadent is the first subject + verb combination, but then we have decem milia as the second subject, and it is clear that the same verb is operative here. We do this in English frequently: “He went to the left, she to the right.” Went is implied in the second part of that sentence. And so, “A thousand shall fall at your side, ten thousand at your right.”