限三晝夜 Three Days And Nights
原來那蓮花座上裝佛祖者乃是個妖王，眾阿羅等都是些小怪。遂收了佛祖體像，依然現出妖身。將三眾擡入後邊收藏。把行者合在金鐃之中，永不開放，只擱在寶臺之上，限三晝夜化為膿血。化後，才將鐵籠蒸他三個受用。原来那莲花座上装佛祖者乃是个妖王，众阿罗等都是些小怪。遂收了佛祖体像，依然现出妖身。将三众擡入后边收藏。把行者合在金铙之中，永不开放，只搁在宝台之上，限三昼夜化为脓血。化后，才将铁笼蒸他三个受用。
逐句解 Line by line
It turned out that the one got up as a Buddha on the lotus seat was a demon king, and the arhats and the rest were all minor fiends.
裝 is the verb for putting on a costume, and the narrator produces it only now, after the fact. Up to this sentence he has described the assembly in his own voice without a word of qualification — which is why the trick has been working on the reader as well as on the pilgrims.
So he put away the Buddha's bodily likeness and showed his demon shape as before.
體像 is a borrowed body rather than a mask worn over one, and 收 is what you do with a magical effect: put it away. 依然 settles the question of which shape is the real one — the demon body is the one he had all along.
The three were carried off to the back and stowed away.
收藏 is a word for putting goods into storage, used here of three men. It stands three times in this stretch, twice of the pilgrims and once of their baggage, and the novel does not distinguish. 藏 reads cáng, to store — not the zàng of the elder's own name, which the same character carries a few segments away.
Pilgrim was left shut inside the golden cymbals, never to be opened, set down on the jewelled terrace, and given three days and nights to turn to pus and blood.
A disposal order in four clauses, and the last of them starts the clock the rest of the chapter runs against: three days and nights to turn to pus and blood. 為 is wéi in 化為, the copula of becoming.
Once he had melted, they would put the other three in an iron cage and steam them for their own eating.
受用 means to enjoy the benefit of a thing, and it is being used here of eating three men. The flatness is deliberate. So is the order of business: the demons will not start on the other three until Pilgrim has dissolved, which is the mistake the whole chapter turns on.
The unmasking is done in a single word. 原來 — it turns out — and four passages of scrupulous description become, retrospectively, a costume. The narrator has been describing a demon king in the vocabulary of a Buddha for three hundred characters and corrects himself without a syllable of apology.
The three days and nights are what make a chapter out of a capture. Pilgrim is not killed; he is scheduled. Everything from here to the end is an attempt to get one object open before a clock runs out, and the demons have obligingly announced how much time there is.
The order of eating is a mistake with a shape to it. They will not touch the other three until Pilgrim has melted — so the safety of the whole party now hangs on the one thing that is not going to happen.