The swarm was a random event, that has since been discontinued. It attacked a player, and the player could not attack back, hitting for one or two points of damage. They would continue to attack the player until either the player dies, or if the player moves a short distance away which would cause the swarm to stop attacking.
They can still be found on the docks of the Piscarilius House, often near the barrels of fresh fish used in gaining favour. They are harmless and will not attack the player, but will follow them within a certain range similar to the chair in Draynor Manor.