Crystal Walkers
The Crystal Walkers are a parasite species that are able to take a wide variety of hosts. They're endemic to Tetra and are most common in central Tetra's deserts and southern Tetra's mountain bases. While generally referred to as Walkers, they're also known as "worms", "the rocks", and other names related to crystals.
If detailed descriptions of parasites bother you, then please skip this article. It's heavy on detail and the details aren't pretty. I find parasites fascinating and I shuddered a few times while writing this.
Physiology
Basics
From the outside, Walkers resemble clusters of crystalline growths on other creatures. The crystals range from turquoise to fuschia in hue, with a violet-pink being the most common color. While they appear solid all the way through, the crystals are only an outer layer excreted to protect the squishy parasite within. One host might be covered in dozens of parasites by the time they notice their infestation, and a serious infestation might number in the hundreds.
The parasites themselves are about a cubic inch (2.5 cubic cm) in size at maturity in the first phase of their life cycle, including the outer crystal covering. They're worm or maggot-like in shape and anchor themselves in the host's body with spines grown as they mature.
While the parasite's outer shell can be cracked off, doing so is excrutiating for both the parasite and their host. It also exposes the parasite, who's often living in a decently-sized hole in the host's flesh. Most hosts refrain from breaking off the shells.
Life Cycle and Lifespan
When an egg hatches on a host's skin, the larval parasite burrows into the skin and begins secreting a crystalline covering to protect itself. This covering often goes unnoticed for days to weeks, as the larval form of the parasite is about the size of a period. The parasites feed on blood in their larval stage, growing rapidly in the first few weeks of life. The host usually notices they've been infested once the outer crystal is large enough to be easily spotted.
Larvae mature into nymphs after about two weeks, at which point they begin producing eggs asexually. These eggs are secreted both onto the outside of the crystal and into the host's body, both of which often spread the parasite's progeny further over and into the host. The parasite spreads onto other hosts via skin-to-skin contact.
The parasite also has cognitive effects once it's spread into the host's body. Hosts are compelled to seek out contact with other creatures, experiencing extreme touch starvation and loneliness when not around others. As a host is increasingly colonized by the parasite, they lose the ability to move as crystal growths block or freeze joint movement from the inside or outside. This process is very painful, but hosts will still attempt to find other creatures to make contact with them.
The parasite has two further life stages that can be reached if enough hosts are in the vicinity of each other. The adult stage requires multiple hosts to be in extended contact with each other. This contact prompts nymphs to develop into adults, and the adults proceed to secrete more crystal to fuse their hosts together. This bonds the adults closer together and enables sexual reproduction, resulting in the final life stage. Sexually-created eggs are laid outside of the fused hosts, and the resulting larvae develop differently from their parents. They grow around the fused hosts, using them as scaffolding to support co-development into a much larger creature. While crystal is still used as armor, it remains mobile by leaving fleshy joints exposed. This multi-parasite creature survives off the remnants of the hosts inside it and seeks new hosts to deposit eggs on, sometimes traveling great distances to do so. Its progeny will grow into the Walkers' nymph form.
The adult stage is rare, as other species generally keep Walker populations under control by killing their hosts when discovered; however, there have been a few instances of unchecked spread.
Nymph Walkers can survive for about two weeks if they're not able to mature into adults. As nymphs die, new larvae move into their shells to begin the life cycle anew for as long as the host survives. Adult walkers can live for as long as they can feed off their hosts, typically living for less than a month of time.
Habitat
The Walkers are endemic to Tetra, though they have occasionally appeared in Una thanks to a prior human expedition that brought them along. They favor hot, dry climates but are able to thrive in all but the coldest areas.
A Walker's primary habitat is its host. Walkers can take a large variety of creatures as their hosts, but exposed skin or flesh is a requirement. As such, humans, Guardians, and Mistfolk tend to be the most-parasitized, with humans being the most common hosts due to their higher population. While Bugfolk have been parasitized in the past, the difficulty of breaking through their chitin limits transmission.
Diet
Walkers in the larval and nymph life stages feed primarily on blood, though they have been known to consume flesh to make room to expand. Small infestations consume a negligible amount of blood, but hosts that have been heavily colonized often suffer the effects of significant blood loss. It's not uncommon for hosts to die shortly after becoming immobile.
Adult walkers feed exclusively on the flesh of the hosts used as scaffolding for their bodies, surviving off them until the hosts are fully consumed. They do not have the ability to feed off external creatures and will die a week or two after their hosts are fully consumed.
Culture
Summary
Walkers do develop a culture of sorts within their hosts. While most of that culture comes down to "spread more parasites", some develop higher awareness and strive to do something more than mindlessly spread their progeny.
Priorities
Most Walkers prioritize spreading their eggs above all else. A small subset of Walkers move beyond this focus and take an interest in the world outside their hosts. These Walkers often learn to manipulate their host's behavior chemically, driving them to their goals without moving a muscle of their own. These hosts often behave as if possessed, and they're quite desperate to see a goal through before the Walker or Walkers driving them die. Two weeks is not a lot of time.
Adult Walkers have similar priorities, but their behavior is inexplicably biased by the hosts they've built themselves around. Loved ones have observed traces of their friends and family in the Walkers containing them even if their hosts have already died. A subset of adult Walkers don't seem concerned with spreading their progeny, instead seeking to complete their last wishes before they're gone for good.
Interactions
Walkers tend to not be interested in each other for non-reproductive reasons. Most communication comes down to "hey, do you know where some open space is so I can lay my eggs?". Self-aware Walkers sometimes do attempt to communicate with their kin, but they quickly give up upon realizing that the creatures they're communicating with will never understand their drive to do something more than reproduce and die.
These self-aware Walkers do sometimes attempt to communicate with other species by manipulating their hosts to speak for them. This is a difficult affair, as the host ultimately still chooses what they say and may communicate poorly. Compounding the problem is the fact that other species see Walker hosts as little more than zombies mocking the people they used to be. Most Walkers are not aware of how other species see them, as their short lifespan has made it difficult to maintain any kind of history of their species, so attempts to communicate rarely end well.
While self-aware Walkers with different hosts have yet to meet, if they did, they'd likely either struggle to get along or decide to reproduce with each other in hopes of creating a self-sustaining lineage of sapient Walkers. Some Walkers do dream of this and choose to infest as many people as possible in hopes of at least one host developing another sapient member of their species.
Child Rearing
Walkers don't care for or raise their children. As soon as their eggs are laid, they're done parenting. As such, Walkers don't understand why other species protect their children, and they don't view children as particularly special. In fact, many expect children to have adult intelligence.
Regional Variations
There's very little regional variation in Walker behavior. Sapient Walkers are the minority among their species, and they've been unable to establish a true culture as a result. Every Walker that develops awareness beyond reproduction is unique in their idiosyncrasies, though the drive to spread remains a struggle.
Language
Walkers primarily communicate with other Walkers sharing the same host. They do so by hijacking the host's blood stream to pass their own chemical messages, enabling them to communicate regardless of their position on the host's body. This allows for simple messages and questions, as well as rudimentary coordination between individual parasites. It also allows for some degree of control over the host's behavior.
A few sapient Walkers have tried to communicate with their hosts. None of these hosts have understood the chemical language their parasites use, but they do understand the effects of that language. Two or three Walkers did manage to establish bidirectional communication with their hosts before dying. Unfortunately, the hosts died shortly after, so any messages passed have been lost to time.
Other Notes
Rates of sapience in Walkers are low. Out of every hundred hosts, two might develop sapient parasites. That said, once a host develops one sapient Walker, more tend to follow on the same host. This sometimes leads to fights over which Walkers get to accomplish their goals before dying. These fights usually mean that none of the Walkers get what they want.