斬妖劍架住 The Prince Caught The Blade
說不了，天王掄過刀來，望行者劈頭就砍。早有那三太子趕上前，將斬妖劍架住，叫道：「父王息怒。」天王大驚失色。噫！说不了，天王抡过刀来，望行者劈头就砍。早有那三太子赶上前，将斩妖剑架住，叫道：「父王息怒。」天王大惊失色。噫！
逐句解 Line by line
Before he had finished, the Heavenly King swung the blade round and brought it down at Pilgrim's head.
說不了 is before the words were out, the standing formula for an interruption. It is doing double work here: Venus is cut off mid-sentence and the novel is cutting off its own passage.
Up came the Third Prince at once, who caught it on his demon-cleaving sword and cried, "Father, stay your anger."
斬妖劍 is the demon-cleaving sword, the son's weapon set against the father's demon-cleaving blade — the two are a matched pair of household equipment, and they are now crossed against each other. 子 takes zǐ, the title reading. 將 is the disposal marker jiāng, putting the sword in front of the verb that blocks with it.
The Heavenly King went white with shock.
大驚失色 is a set phrase and still in daily use: startled out of one's colour. English says went white for the same thing, and gets there by the same route.
Ai!
One character, standing as its own sentence, and it does what a drum-beat does. The novel is about to stop the action entirely for a hundred and fifty characters.
The rescue is a sword-block by the accused man's son, and it is the most surprising thing that has happened in the chapter. Nezha is not defending Pilgrim out of friendship — the two of them fought a war — and the novel does not tell you his reason for another two passages.
The Heavenly King's reaction is the giveaway. A father whose son stops him would be angry. This one goes white, and the whole digression that follows exists to explain why.