To unlock this achievement, you need to travel to Hallowed Marsh and find a swarm of fireflies. They are relatively common throughout the marsh, so shouldn’t be difficult to find. Once you approach them, they will encircle you and will not leave until you find a Firefly Plant. They are always nearby to where you found the fireflies, but can be found throughout Hallowed Marsh with equal commonality if you’re in a rush and need to leave the region. Once you find one, stand on top of it until it eats the fireflies. When it’s done eating, it will spit out some Firefly Powder. Pick this up to unlock the achievement.