Final Answer:
The figurative language device used in the phrase "He strode in a swarm of fireflies" is a metaphor. It describes the person's movement with the vivid imagery of fireflies surrounding them.
Step-by-step explanation:
A metaphor is a figure of speech that involves implying a comparison between two unrelated things, asserting that one thing is another. In this case, the person's stride is compared to the movement within a swarm of fireflies. The metaphor creates a vivid and imaginative image, suggesting that the person's presence or walk is surrounded by the flickering and glowing quality associated with fireflies. It adds a layer of poetic richness to the description, evoking a sense of enchantment or a magical aura surrounding the individual.