From this one word of Tripitaka's, ill luck was upon them.

晦氣 (huì qì) is bad luck of the settled, sticky sort — the 晦 is the dark of the moon, the last night of the lunar month. The narrator stops the action for one sentence to tell you that this is where it all goes wrong. And note where he puts it. Not at the blow. At the decision to believe Bajie.

Sure enough, taking that idiot's slander for truth, he pinched his fingers into the sign and began to recite the spell.

捻訣 (niǎn jué) is pinching the fingers into a ritual sign — the gesture that arms the spell, like keying in a code before pressing send. Then the recitation. The 緊箍咒 tightens the gold band round Wukong's skull, and there is no defence against it of any kind. (那 points, nà; 子 is the toneless suffix; 手中 takes 中 as zhōng.)

Pilgrim at once cried out, "My head! My head aches!

Two words, said twice. The creature who fought heaven to a standstill is reduced to "my head, my head" — and the thing doing it is not a weapon or an army but an elderly monk muttering. This is the only power on earth that works on him, and it has just been used to punish him for being right.

"Stop, stop! If you have something to say, say it."

莫念: stop saying it. Then, "if you have something to say, say it" — which is Wukong asking for a hearing while in agony. The strongest of the four is begging the weakest, and the weakest is the one holding the whip.

The Tang Monk said, "What have you to say?

Curt, and the opening of a prosecution rather than a conversation. (甚 is shén.)

"A monk must be merciful at every moment, keeping a heart of goodness in his every thought — sweeping the floor for fear of harming an ant, and screening the lamp with gauze to spare the moths.

The couplet is famous, and it deserves to be: sweep the floor gently in case there are ants beneath it, and put a gauze shade on the lamp so that the moths cannot fly in. That is the standard of mercy being invoked — a compassion so fine-grained that it takes account of insects. As an ideal it is beautiful. Held up here, against a blow that killed nothing whatever, it is being used to convict an innocent man. The elder has never been more admirable, or more wrong, than in this sentence.

"How is it that you do violence at every step, striking dead this innocent, ordinary person? What use would there be in fetching the scriptures?

平人 is an ordinary person, a civilian, somebody with no part in any of this. And the argument is a real one: what would be the good of fetching scriptures with a killer in the party? In the abstract it is unanswerable. Here it is aimed at a man who has killed nobody. (行 is xíng, "to act"; 將 fronts the object.)

"Go back where you came from."

Four characters. Go back. The one person who can see what is hunting them has just been sent home.