Resembling scuttling crustaceans or arachnids, the Swarm will rush to the nearest corpse and burrow into it, reanimating it and converting the corpse into a Fodder or a Slasher. The Swarm allow for multiple bodies to be infected simultaneously to cause a quick wave of Necromorphs that can overpower the player quickly if not dealt with fast enough. If the reanimated body is destroyed, the Swarm will rush over to the next available corpse and re-animate a new body until killed. Quickly dismember the body before the Swarm gets to it, or destroy them before they can reanimate a body for better breathing room.
The Swarm seldom attack alone. If there is one Swarm member, there may be others. It is recommended to eliminate the Swarm before it is capable of reanimating or infecting a corpse. As with all Necromorphs (with the exception of the Regenerators), escaping from the Swarm will usually prove futile, as they will quickly find a body and chase after you, if they have not claimed one already.
Be careful about dismembering every corpse in a seemingly Swarm-free room, as when deprived of viable corpses, Swarms will target Isaac and Carver instead. On Impossible difficulty, being attacked by a large group of these enemies can quickly prove to be fatal.
The best course of action is to wait for Swarm creatures to latch on to a nearby corpse, then obliterate it with heavy firepower, destroying the corpse as well as any Swarm members infesting it.
The process of infecting a corpse can be carried out by a single Swarm member if given the time. This makes the use of weaponry that can make short work of a large area at once an advisable option.
As with the Infector, dismembering any corpses in the room will prevent the Swarm from reanimating them (resulting in them targeting the player), or will result in the production of weaker Necromorphs.
The player can predict Swarm attacks as they may encounter a group of corpses on the ground that cannot be moved by Telekinesis, similar to when other Necromorphs play dead.
Similar to Creeper heads, if the Swarm infests a corpse that is under Stasis, they will repeat the burrowing animation endlessly until the Stasis effect wears off. Use this chance to pick off individual members or the whole group if possible.
The Swarm can, much like the Swarmer, attach itself to Isaac and eat away at his health. If Isaac's health runs out while they are attached to him, they will crawl all over him, one eventually finding its way underneath his helmet. Isaac tears his helmet off to fight off the creatures, until one enters his mouth and goes inside his body. Isaac coughs a bit, confused, before falling to his knees and vomits a massive amount of blood. Then, with a sort of growl, Isaac stands up again, his eyes and mouth foaming, similar to a Fodder, now having become a Necromorph and lumbers off.
The death scene for this type of Necromorph is the second one in which Isaac turns into a Necromorph, the first being the Divider head component.
If a Swarm kills Isaac or Carver in an area with no air, they will pull their helmet off even though this would kill them, however, the death scene still plays as normal.
It is worthy to note that even if an area is cleared of all but one Swarm member, Isaac will still be attacked by several in the death scene.
Like the Ubermorph from Dead Space 2, the Swarm bears little resemblance to other Necromorphs; its skin looks much smoother and more alien than the other obviously human based Necromorphs. A possible explanation for this is that they are created directly from Corruption. This can be deducted from a seemingly limitless supply of Swarm members that a Guardian can create.
The Swarm seem to have traits like those of octopi, being able to squeeze their bodies and make themselves smaller, slipping into cracks and holes. Because of the size of the creature, Isaac would mostly choke to death rather than the creature going into his body and killing him from the inside out. This is improbable, however, due to the large volume of blood that Isaac/Carver expels during the death-scene.
The only Necromorphs produced by the Swarm in-game are Fodders and Slashers, but the creation of Fodders is more common.
Sometimes, even after completely dismembering a body, it will not stop the Swarm from infecting corpses; it will simply cause the corpse to generate new limbs.
The Swarm may have been inspired by the Flood from Halo. Due to way they behave and the way they are designed.