You can use this as a side quest or ramp it up to work as- or along with- your main story.
One PCs start feeling depressed. There’s no obvious reason for the depression, and it offers no mechanical penalties. However, thrilling and joyous events feel bland and muted. This continues, getting worse but still not offering any true mechanical penalties. It is all RP-related.
The true reveal comes when something great happens but the sad character doesn’t actually see it. The affected player sees a stripped down version of the event. It isn’t what everyone else sees.
While inspecting the sad character, the other PCs will notice something strange about their eyes: their pupils are not fully black. Around the edges, other PCs can see tiny, triangular shapes pointing in toward the center of the pupil. Studying them, they can see that they are moving slightly and barely protruding from the eye. After closer inspection, they see that the triangles are teeth, biting into the eye from the inside and the pupils are actually the throat of a small creature.
The creature is a very tiny parasite. It burrows into a hosts eye until it can bite down and plant itself there. Its long, worm-like body reaches back through the eyeball, snaking around the optic nerve.
The creature feeds on visual input and the emotions that come with them. At first, the images filtered through the parasite lose their emotional resonance, sucking the joy out of whatever the character sees. The mid-stage is where it will eat actual information (thus the host character not seeing what truly happened). In the later stages, it will eat enough visual input and have enough control to alter what they see even more. Before the end, the host will start going blind to specific things, such as living creatures and distant images before going completely blind.
The parasites in the PCs’ eyes are smaller versions of the mother parasite. The mother parasite is a slime-covered, twenty feet long worm with a large circular mouth full of teeth, no eyes, and rolls of flesh that tighten and expand so it can move. It has three rows of circular spines; one behind its mouth and two near the center of its body. It can use those to grip walls to climb or for extra damage when it thrashes to attack.
The small parasites psychically feed the visual information to the mother parasite. The mother parasite resides deep within a cave. She grows more powerful as the small parasites spread and feed.
If you are using this as a side quest, you could tie the location of the cure into your story. This can get players to CHOOSE to go where you want them to. It could send them to find an expert surgeon who they think can cut it out or a grand alchemist who they believe can brew an elixir to kill it. These characters could have crucial plot points for the next part of your adventure.
But if you want to get really interesting, don’t make the cure that easily obtainable. To ramp it up further, have the other PCs learn that they are infected as well, feeling the first stages of the parasite’s effects.
With everyone infected, PCs have the looming doubt that what they are seeing could be false or altered in some way. For example, when fighting a gang of goblins, one PC could see a goblin behind another character. The PC goes for the goblin and hits it. All other players see the attacking PC hit the player instead. The goblin was an illusion that only that one PC could see.
As they get closer to the lair of the main parasite, its ability to alter their vision increases. And with all of the PCs infected, it will show illusions they ALL see. It will attempt to lead them away from its lair. It will have them walking away from the lair while seeing themselves delving into the dark cave that leads to it. It will manifest illusionary creatures for them to attack or run away from. Its primary ability is creating illusions, so it will do all that it can to keep them from finding it in the cave.
How to defeat the illusions
The key for players to figuring out what is real and what’s an illusion is spotting inconsistencies. The players have to communicate, noticing and explaining what they see. When they spot inconsistencies, that’s a clue that what they are seeing isn’t real. Perhaps one goblin has a bandoleer to one of them but not the other. Perhaps the turn in the cave isn’t as sharp to all of them. The main parasite can project a basic image, but not a perfect duplicate. The minds of the hosts will always fill in some details.
Players need to communicate, focusing together on specifics so they can decipher what is real and what isn’t. Don’t just rely on high perception checks to determine if they notice something.
When players are working together to figure out what is real, be sure to throw in some threats (real and illusionary) that require quick thinking. A flash flood in the cave, a forest fire, or a circling pack of wolves could all be real or false threats. The flood and fire would both be difficult to compare, given that they are constantly moving and altering in appearance.
Battling the mother parasite
When the PCs finally get to the mother parasite, the battle is just as much about misdirection as it is combat.
The mother parasite will use its powers to not only deceive the PCs by altering what they see. It will make an illusion version of itself for the PCs to fight while it climbs the wall and hides from them, clinging to the ceiling of the cave. When they attack the illusion, it will appear to suffer and die.
If that doesn’t work, it will make illusions of walls to misdirect players, manifest false threats, and block itself from the PC’s vision to the best of its abilities.
If the mother parasite can’t hide, it will cause the PCs to see an illusion of the parasites bursting through their own eyes. The blurry, darting, snapping parasite will take up almost all of the PC’s vision. PCs with this happening to them receive a penalty to their attacks.
Like the small parasites, the mother parasite’s bite has a numbing quality to it. Those bitten by the mother parasite stop feeling anything less than extreme pain and don’t take any penalties because of pain. However, as their bodies go numb, they will find it difficult to walk and use weapons/tools properly. They take a penalty on their speed and attacks.
While it has a bite attack, and it can thrash about trying to gore with its spines, its main defense is the illusions. Its size and only moderate movement speed make it an easy target to hit, and its grub-like skin does not offer much protection.
When the mother parasite loses just over half its health, it will lash out more, physically and with more intense illusions. It fills the PCs vision with an onslaught of pulsing, nearly nonsensical images. Flashes of fire, enemies, family, different surroundings, heaven, hell, whatever it can use to distract them. Its final hope is to drive the PCs mad or create enough mental strain to force them to flee or give it enough time to escape.