我有仙方 An Immortal Prescription
他說：『我有仙方，止用小兒心煎湯為引。』是朕不才，輕信其言，遂選民間小兒，選定今日午時開刀取心。他说：『我有仙方，止用小儿心煎汤为引。』是朕不才，轻信其言，遂选民间小儿，选定今日午时开刀取心。
逐句解 Line by line
He said, 'I have an immortal prescription. It needs only the hearts of small children, decocted into a broth, to serve as the conductor.'
為引 takes wéi, the copula — to serve as, to act as — not the wèi that means "for the sake of." 煎湯 is the ordinary verb for preparing Chinese medicine: simmering ingredients down into a decoction. Everything in the sentence is standard pharmacy except the ingredient.
It was my own worthlessness that believed him so lightly, and so I had small children selected from among the people, and fixed on noon today for the knife to open them and take out the hearts.
開刀取心 is put with no softening at all. 午時, the noon double-hour, runs roughly from eleven to one; executions were traditionally held then, when the 陽 force was strongest, so the timing is not casual. The king had scheduled it like a state execution.
The prescription is the hinge of the whole two-chapter episode, and it is quoted in the demon's own words — the only time in the chapter we hear him speak directly about his plan. Fifteen characters, and they are all technical.
That is what makes it work. He does not argue that children's hearts are powerful or that the king deserves to live. He states a formula and names a missing component, in the register of a man reading off a prescription pad, and leaves the king to do the arithmetic himself.