While parasitic symbionts afflict their hosts with adverse side effects, most hosts believe the benefits outweigh this. Sometimes such hosts band together to form secret cults based on their common infestation. These cults are often interested in prolonging the life of their parasites, acquiring more parasites, or even spreading parasites to others. Such groups might comprise the elite or secret warriors of a powerful political or religious sect. Fanatics may even conceive their parasites as divine blessings and hosts as carriers of the living embodiment of a deity or demon. Depending on how a host contracts the parasite, the cult may perform special rituals coinciding with the parasite’s reproductive cycle or unique holy sites where followers make pilgrimages to contract the parasite.

Depending on how individuals view parasitic hosts, the distribution of a parasite might be illegal. Many view hosts with suspicion, and some even face persecution. Alternatively, parasites occasionally turn up as expensive items on the black market. While possession of a parasite is not necessarily illegal, most cultures frown upon parasite use, and respectable merchants will not openly sell them.

Chlor Algae

This symbiotic plant grows within the lungs of living creatures by feeding off exhaled carbon dioxide, which rapidly converts to oxygen. The algae allow the host to breathe safely in hostile environments and act as a filtering system to remove toxins entering the host’s body through the lungs.

Outside of a host, chlor algae grow deep underground in stagnant pools. It is perennial, going dormant each year during the cold season.

Appearance. The eyes and skin of a chlor algae host assume a faint bluish-green tint.

Special Abilities

Breath Spores. Once per long rest, the waste produced by the parasites must be purged, during which the host exhales a small cloud of toxic spores in a 15-foot cone. Living creatures caught in the cone must make a DC 13 Constitution save, taking 3d6 poison damage and being poisoned on a failed save or half as much damage and no other effect on a successful one.

Sense Water. The host can use an action to sense the location of any water within 120 feet.

Symbiotic Immunity. The host is immune to all inhaled poisons.

Water Breathing. As long as the chlor algae remain in the host’s lungs, the host can breathe underwater.

Disadvantage

Lung Spread.If left untreated, chlor algae spread throughout the host’s lungs, slowly impeding their ability to breathe. Each day the algae spread, the host gains one level of exhaustion. Destroying the algae ends the exhaustion. Alternatively, a host can reduce the parasitic spread by inhaling the fumes of a special alchemical incense. Crafting the incense requires a successful DC 14 Wisdom (Medicine) check. It takes 12 hours and 10 gp in materials to produce one 30-dose incense block.

Nerve Leech

A nerve leech is a slug-like parasite that lives at the base of its host’s spinal column. It feeds off the life force within the host’s body while interacting with the nervous system, stimulating the host and triggering a state of heightened awareness. Their secretions also stimulate the metabolism.

Host Appearance. All the body hair of a nerve leech host bristles and stands on end, and they constantly twitch and blink. Their pupils take on a reddish hue.

Special Abilities

Increased Metabolism. The nerve leech causes the host’s blood to clot quickly. They have advantage on death saving throws.

Symbiotic Awareness. The host has advantage on all Dexterity saving throws and on Wisdom (Perception) checks.

Symbiotic Resistance. The host is immune to paralysis and poison and to any spell or effect that would alter their form.

Disadvantage

Rapid Aging. A nerve leech causes its host to age three times faster than expected.

Soul Maggot

A soul maggot is an incorporeal astral denizen comprised of divine energy that manifests in the soul of a living host. These parasites form from the thoughts, memories, beliefs, and ideals that escape from a sentient being just before it dies. Once the soul maggot merges with the host, the host gains the symbiotic ability to access realms of knowledge otherwise unfamiliar to them. In some instances, a soul maggot may become infused with thoughts pertaining to a task left unfinished by the previous host. If so, the soul maggot compels the new host to complete the task—individuals contract soul maggots through exposure to other soul maggot hosts. The maggots do not reproduce naturally and have no incubation period. If an infested creature dies, their soul shatters, and all humanoids within a 30-foot radius must make a DC 12 Wisdom saving throw or become infested with a soul maggot. A soul maggot survives within its host for approximately three years. When it dies, the host suffers 3d6 psychic damage. Casting banishment upon the host also destroys the parasite.

Appearance. The pupils of a soul maggot host dilate excessively, giving them the appearance of having no iris. They also emit tiny cracks of static whenever first touching another living creature.

Special Abilities

Symbiotic Knowledge. Once per short rest, the host can call upon the knowledge of the symbiont. They have advantage on their next Intelligence check.

Symbiotic Resistance. The host is immune to being charmed.

Telekinetic Blast. The host creature can concentrate the soul maggot’s energies into a potent blast of telekinetic force in a 30-foot radius centered on the host. Targets must make a DC 14 Dexterity saving throw, taking 4d6 psychic damage on a failed save or half as much damage on a successful one. After using the blast, the host gains one level of exhaustion.

Spirit Worm

A spirit worm is an undead parasite made up of a worm-like strand of necrotic energy that houses itself in the host’s heart. This attunes the host to necrotic energy as the parasite simultaneously feeds off their soul. The host also becomes more aware of necrotic energy sources and resistant to necromancy.

Individuals can contract spirit worms through physical contact with infected undead. The parasite reproduces by seeding the undead with its offspring, which mature within the undead before seeking a humanoid host. After living in a host for two years, the spirit worm leaves and seeks an undead creature in which to lay its eggs. After laying its eggs, the spirit worm dissolves into pure necrotic energy. Whenever a living creature comes in contact with a spirit-worm-infested undead, it must make a successful DC 13 Constitution saving throw to avoid becoming infested. If the individual fails, the spirit worm larva buries into the host and incubates over the next 2d6 days, morphing into a new symbiotic parasite.

A cleric can destroy a spirit worm by using Channel Divinity to turn undead. Destroying the parasite also deals 3d6 psychic damage to the host.

Appearance. A spirit worm host’s skin possesses a grayish tint, and a dull-yellow film forms over their eyes.

Special Abilities

Necrotic Energy Burst. Once per long rest, the host must purge the buildup of necrotic energy within their body. The host releases this energy in a 30-foot-radius burst centered on themselves. Creatures in the area must make a DC 13 Constitution saving throw, taking 4d6 points of necrotic damage on a failed save or half as much on a successful one. On a failed save, the target also reduces its hit point maximum by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

Symbiotic Immunity. The host is immune to necrotic damage. In addition, the host gains 5 temporary hit points each morning.

Disadvantage

Taxed Stamina.Hosting a spirit worm is physically draining. The host has disadvantage on any saving throws to avoid exhaustion.

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