If you mean figurative language then
"air of night"
because the audience would be familiar with them as parables were highly popular then. Also, it being a monk, it is supposed to be a serious story that teaches you something, not a funny or obscene story such as those from the Wife of Bath or the Miller.
True, the speaker in the poem, a farmer, is concerned with loyalty: He is struggling to decide how much loyalty he owes a none-too-reliable former employee who has returned in his absence and whom his wife says is dying