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Building the Swarm

Source Starfinder #43: Icebound pg. 52
The most terrifying aspect of the Swarm isn’t its ruthless consumption or alien outlook—it’s that the Swarm is virtually unstoppable. It overcomes obstacles by quickly developing an organic response. Against physical barriers, it excretes corrosive acid. Faced with thinking opponents, it deploys debilitating psychic powers. When starships threaten to stymie its invasions, it forms massive living starships of its own. Individual Swarm creatures are called components for good reason: each one is a singular element of a constantly adapting hive mind. The Swarm is thus as dangerous as it is adaptable—and it is very, very adaptable.
You can bring this same terrifying adaptability to your gaming table by inventing new Swarm creatures to plague your players. Details on how to create new aliens appears on page 126 of the Alien Archive, but there are several additional considerations when you’re making a new Swarm creature.

Appearance

Source Starfinder #43: Icebound pg. 52
Swarm creatures are biological; they’re living things, even though they don’t value independence or self-sufficiency as living organisms typically do. Most Swarm components are insectoid in appearance, with segmented carapaces, vicious pincers, and other features reminiscent of insects or arthropods. They might have a stance that’s more like a humanoid creature, with two arms and two legs, but this should amplify their alien nature when topped with mandibles, antennae, or other distinctly insectoid features.
Many Swarm components have membranous wings, but not all of them can fly. In many components, but particularly in large ones, these wings are vestigial and lack the strength to lift the creature in the air. Swarm components usually have a tough exoskeleton, often with intimidating spikes or barbs. Their internal biology varies based on necessary function and form, but components nearly always contain acidic internal fluids. Even those Swarm creatures that don’t rely on acid attacks nevertheless have corrosive ichor, which might dissolve the body quickly when defeated, or even eat through a floorboard or deck plate.

Mannerisms

Source Starfinder #43: Icebound pg. 52
A Swarm component is almost never encountered alone. The PCs might get into a fight with only a single Swarm creature— such as when they want to eliminate a forward scout or overcome a tough guardian—but there are always other Swarm creatures nearby. The Swarm shares a hive mind, giving components a unity of purpose. This hive mind controls every aspect of a Swarm component’s existence, and thinks nothing of sacrificing numerous components to achieve a goal. The Swarm components themselves think nothing of being expended, as their individual will is subsumed with the hive mind.
This hive mind is strongest when Swarm components are within 30 feet of each other; at this proximity, they instantly share threat assessments and bolster each other’s mental fortitude. Swarm creatures this close to one another should always display an eerie uniformity—they walk in lockstep, turn their gaze in unison, and so on.
Swarm components work naturally as a team, and their tactics should reflect this. They typically seek to flank their foes to make them flat-footed, while those with natural ranged weaponry might provide covering fire or harrying fire to support melee fighters. Smarter components might direct weaker creatures to provoke attacks of opportunity during a retreat, to better ensure the survival of stronger Swarm.
Some Swarm components, and particularly those suitable as foes for mid-level or high-level PCs, hone their telepathic sensitivity into spell-like abilities. The spells Swarm creatures cast should vary, but they nearly always resemble psychic powers in their manifestations and effects.

Swarm Mechanics

Source Starfinder #43: Icebound pg. 52
When building a Swarm component using the monster creation rules (Alien Archive 126), keep the following points in mind.
Array: A significant majority of Swarm components use the combatant array, as they are tough creatures that rely on brute force. Specialized components might use the expert array, and only components that rely on psychic powers use the spellcaster array.
Alignment: All Swarm components are chaotic evil. Even the hive mind’s most calculated strategists pursue rapacious and destructive goals. They aren’t beholden to a sense of fair play and follow no authority other than the hive mind.
Creature Type: Swarm components are always monstrous humanoids, regardless of their appearance or function.
Creature Subtype: The swarm subtype applies to clouds or creeping collections of very small creatures; it doesn’t represent Swarm creatures. A cloud of tiny Swarm components might congregate as a swarm and have the swarm subtype, but there isn’t a specific subtype for Swarm components.
Other Grafts: Swarm components virtually never have class grafts, and only the strangest of circumstances would result in one with a template graft.
Senses: Swarm components bear antennae that allow them to detect subtle atmospheric changes. All but the weakest components have blindsense (vibration) out to 30 feet, and powerful ones might have blindsense out to a longer range.
Defensive Abilities: All Swarm components belong to the hive mind, and each has the following universal creature rule.

Swarm Mind (Ex) Members of the Swarm are bound together into a singular hive mind by a blend of exuded pheromones, imperceptible movements of antennae and limbs, electrostatic fields, and telepathic communication. All Swarm creatures within 30 feet of each other are in constant communication; if one is aware of a threat, all are. (Such awareness can spread along a chain of Swarm creatures under appropriate circumstances, potentially alerting distant Swarm creatures.) In addition, once per round when within 30 feet of another Swarm creature, a Swarm creature can roll twice and take the better result on a saving throw against a mind-affecting effect.

Immunities: All Swarm components are immune to acid and fear effects. Their own bodies circulate acid, so caustic chemicals don’t affect them, and the hive mind has no place for fear, having excised it entirely. These immunities, along with the Swarm mind defensive ability, are grouped together as a single special ability when assigning special abilities to a monster you’re creating. It’s not unusual for Swarm components to have other immunities (or resistances), depending upon their function.
Languages: Swarm components speak Shirren (or, perhaps more appropriately, shirrens speak the original language of the Swarm) and have telepathy out to 100 feet.
Special Abilities: Swarm special abilities generally fall into one of two types: physical or psychic. Physical abilities are those resulting from the component’s unique physiology, such as the arm-mounted acid cannon of a corrovox, the vicious arm blades wielded by a thresher lord (Alien Archive 110), or the trepanning spike of a mindreaper (Alien Archive 2 122). Many Swarm components inflict poison with stingers or barbs. Ranged physical attacks most often deal acid damage, but this isn’t a rule; a nauphage (Starfinder Adventure Path #20: The Last Refuge 60) fires globs of electrically charged adhesive. Psychic attacks need not be represented with spells or spell-like abilities; many Swarm components can assail the minds of their foes with psychic abilities that are powerful in effect but limited in scope. Examples include the corrovox’s disorienting mental chatter, the synaptic shriek of a psychovox (Starfinder Adventure Path #24: The God-Host Ascends 59), or the parasitic psychic siphoning of a xersk (Alien Archive 3 106).
Spells: Those rare Swarm creatures with actual spellcasting have spell-like abilities rather than spells like a spellcaster PC. Their spell-like abilities should include—and perhaps be limited to—mind-affecting spells. Spells that aren’t mind-affecting should fit the specific component’s theme; for example, the xersk has numerous staring eyes and a master Perception skill, so it also has see invisibility as a constant spell-like ability.

Swarm Starships

Source Starfinder #43: Icebound pg. 52
Like most biological creatures, the Swarm must use starships to travel through the vacuum of space. While such starships are occasionally stolen from defeated races, more often they’re Swarm components bioengineered to an immense size. These living starships carry hordes of Swarm creatures from system to system, invading and devouring as they go.
Swarm starships should, like all Swarm creatures, have an organic and insectile appearance, with overlapping carapace plates as durable as starship bulkheads. The process for building spacefaring starship creatures is described on page 126 of the Starship Operations Manual, and you should keep the following points in mind when designing these creatures.
Creature Type Graft: Swarm starships, like smaller Swarm components, are always monstrous humanoids.
Special Abilities: All Swarm starships gain the living starship special ability and the void adaptation universal creature rule for free. Other special abilities you might choose include death throes, speed burst, and titanic slam (to represent a gnawing bite or ripping claw).
Swarm starships are part of the overarching hive mind, though the effects are different on a starship scale. All Swarm starships gain the following special ability for free.