Basic melee minions. Fast but weak. Good at surrounding enemies.
Book of Dagon
The most common of the unspeakable arthropod horrors lurking in the sea. They are that which hides in seaweed; They are that which snaps at you as your toes try to find the bottom; They are the antediluvian vermin that scurried through old surface worlds which now lay buried beneath the sea.
The Swarm are the first minion you get in the game, and individually the weakest. They make up for this however by being the easiest to get by far. You summon 10 of them at a time, and breaking shaking barrels will give you several Swarm for free. Swarm are mostly outclassed by Crabs, who come in similar numbers and don't die quite as easily, but Swarm have the advantage in damage dealt.
Still, you're likely to never not have some Swarm in your army, and even when playing as an Apostle who cannot directly summon Swarm you can often find yourself gathering them anyway.