With my favorite month, October, coming up, I've been thinking a lot about villains, monsters, and Big Bad Evil guys. Admittedly, that's partially because I've been writing evil villains for the Evil Archetypes book (releases on Patreon Saturday!) But, in a broader sense, monsters are always on my mind; they're the DM's most often used resource, and one of the biggest ways that you can interact with your players. It always pays to have a monster in mind when you start the adventure.
My all-time favorite monster incorporates a little bit of Lovecraft with a lot of roleplay and body horror. I've covered them before, and graced them with a 5e monster stat block. Allow me to introduce the Tsochar, or plural, the Tsochari:
Slimy bundles of tendrils and nerves, you wouldn't expect the tsochari to be a dangerous race. In truth, by looking at one, you probably couldn't tell how it even moves about or how it might defend itself. It has a central bundle of muscular strands, but nothing that resembles limbs, a torso, or a face. Of course, you probably won't be coming across a tsochar 'naked' as such very often -- the tsochari live inside other creatures.
The tsochari are parasites which wear their hosts like meat suits. After crawling inside their host's body, they anchor themselves to the skeleton and nervous system, disappearing completely between the body's cavities. Then they choose to inhabit or replace the host. Because the host can resist tsochar influence, the invader normally seeks to subjugate the host, telepathically issuing commands that the host must follow, and wracking it with pain if it disobeys. The tsochar drains the host of all vitality, and without curative magic, the host normally dies within a few weeks. Even with curative magic, it's rare for a tsochar to remain for longer than a year or two, lest it be detected, so the host's days are clearly numbered.
Failing this, the parasite can simply kill the host and replace its nervous system, essentially piloting its lifeless body like a puppet. The body remains alive, and the tsochar begins to devour it from the inside out. No curative magic can sustain a body in this way, so the body is normally hollowed out within a few months, forcing the parasite to move on.
The tsochar come from a cold, dying world, far out among the stars. There, the sun is little more than another bright star in the endless night. The reason, and indeed the means, by which they come to our world is mysterious. The tsochar represent a threat of invasion and assassination, not of outright colonization and genocide.
When you use them in your campaign, they make the perfect recurring badguys. They're excellent masterminds, and can be literally anywhere or anyone. They prefer authority figures, people with power that they can use against the player characters. If discovered, a tsochar can escape and quickly replace a new host, disappearing into a crowd, and jumping three more times before day's end, completely vanishing from the party.
When you encounter a tsochar, it's always initially a roleplay encounter, not a combat one. The party needs to deduce that the individual they're talking to is acting strangely. If the party is familiar with the individual, and he or she has been replaced by a tsochar, it will have no memory of meeting them, and will be very avoidant of questions or investigation. If the individual is being inhabited, it might seem sickly, afraid, and present a variety of nervous ticks.
If the party does realize a tsochar is present, there might be a hostage situation at play, if the tsochar is inhabiting someone alive. They're presented with the unique challenges of extracting the parasite, and somehow finding a way to trap it once it's loose.
For Evil DMs
My all-time favorite trick with the tsochari is to secretly implant one inside a player. If the parasite is loose, and the party splits up to find it, I'll simply have it ambush one party member in secret, inhabiting it for the rest of the session. When the party at last feels safe, the tsochar forces the party member to start strangling his friends, or hacking them up with swords. Real confusion, every time. Priceless D&D, right there.