A parasitic mold, the Siren Fungus has several forms throughout its lifecycle. It is often encountered as a translucent, gelatinous orb, softly glowing with colored light. These are found in the lairs of predators, most especially giant arachnids. The fungus has a strong psychic effect in a wide radius, drawing sentient creatures to it. The spider often eats these sentients, but eventually someone will manage to grab it and try to carry it off as a gleaming treasure, only to have it burst in a spray of fluid. This spore-bearing fluid eventually finds its way into the bloodstream, where its next phase of life begins.
The spores eventually reach the brain, where they latch onto the hypothalamus and begin eating and growing. The host will lose control of their appetite, eating ravenously and unceasingly. The host’s mood is also affected, becoming more aloof and short-tempered. After an incubation period of one to three months, the mature fungus releases chemicals into the hypothalamus that remove any sense of fear. The host will find itself drawn to adventure and danger, often leading it to explore caverns, swamps, or other habitats of apex predators. Eventually, the host is killed – often by a monster like a giant spider. The death of the host triggers a reaction in the fungus, breaking its structure apart and spreading through the whole bloodstream. As the spider feasts on the host, the fungus enters its body and the next stage of life begins.
The fungus enters a second spore stage and finds its way to the reproductive system of the arachnid. In females, it consumes the ova and forms its own fungal sac-like orb. When the female lays this sac, she treats it as her own and places it securely in her web. In males, the reproductive system is consumed and the fungus forces its way out of the male; the castrated male then treats the “egg sac” as a female would. The sacs start as small, colorless balls, but immediately begin emanating a psychic field which draws creatures to it. When the spider eats these creatures, the fungus extends tendrils to consume whatever remains the spider leaves behind, and gradually grows in size and psychic strength. Thus the lifecycle begins again.
Chirurgeons who had observed the fungus note that its mature phase can take years to fully grow, during which the female will eventually regrow her own eggs and continue her lifecycle, leaving the fungal orbs undisturbed. The relationship is symbiotic, bringing prey to arachnids in places with few usual visitors. Diagnosis of a host is difficult to distinguish without an autopsy or cranial incision. Treatment consists of fasting to starve the fungus, and a variety of herbal medicines with anti-parasitic properties.
An unknowing host could lead a party of adventurers to potential doom. That paunchy, fearless knight they meet in a tavern, hiring them for an expedition into dangerous caverns, may well be taking them to a giant spider’s lair. Beware those gleaming cave jewels.