Brood Leech Swarm

Most smaller species of leeches do not tend to swarm, but brood leeches are prone to gathering in seething, undulant mats of squirming gluttony. When they gather in sufficient numbers to swarm, they eschew the stealth of a lone leech's feeding methods in favor of swift and merciless feeding. In these situations, their mild venom can affect much larger creatures than their usual prey.