Final answer:
The sentence conveys a tense mood through the use of vivid language that depicts fear and suspense, highlighted by the characters' reactions to a horrid sound.
Step-by-step explanation:
The mood of the sentence in question is tense. This tension is conveyed through the depiction of Darzee and his wife cowering in the nest, indicating fear, and the description of a "low hiss" that is characterized as a "horrid cold sound." Such vivid language serves to create a sense of suspense and imminent danger, which causes the character Rikki-tikki to react by jumping back. This response underlines the intensity of the moment, emphasizing the mood of tension that pervades the scene.