怕讖語 The General Fears the Prophecy
老妖聽言，大驚失色。這正是「大將軍怕讖語」。他聞得自家人這等說，安得不驚。老妖听言，大惊失色。这正是「大将军怕谶语」。他闻得自家人这等说，安得不惊。
逐句解 Line by line
Hearing this, the old fiend was badly shaken and the colour went out of his face.
大驚失色 — greatly startled, losing colour — is a set phrase and still current. Eight characters ago he was planning a meal.
This is just the case of the great general who fears the prophecy.
將 is jiàng, a general — the rank. 讖語 is a prophetic saying, the kind that comes true because it has been spoken. The narrator steps out of the scene to name a proverb, and the proverb is about how a commander can be defeated by a report.
Hearing one of his own people talk like that, how could he not be shaken?
聞得 and 安得 put 得 at dé both times — after a verb of hearing, and in the question frame how could it be. 自家人, one of his own, is doing the work: the report is frightening because it is not enemy propaganda.
The narrator names the proverb rather than explaining the moment, which is this book's habit. A general who fears a prophecy has already lost something, and what this one has lost is his appetite.
And the last three characters give the reason. It came from one of his own. The imp who wept and laughed has, in the space of one conversation, taken the cave's plan away from its commander — and the imp who does that next will hand him a better one.