The key is to make the thing violate some element of "humanity" considered "normal" by you and/or your players as actual people.
Here's an example, and then you get to make your own!
* English, an NPC thief a PC has been crushing on, says, "Hey, you heard about those Tormundrs or whatever? I heard they only eat butter and only drink cocktails! [not true!] You wanna go with me to one of their rallies? Sounds like a blast."
* At the rally, a priest with a candle-hat and a body like a twig executes an entire sermon in hums while the crowd just sways and hums back. "What? You're not feeling it? Oh, that's because you've eaten today."
* Say, hey, PC, you remember that rally for the Tormundrs you went to? How they fast all the time? I'll give you 25XP if you don't eat until sunrise."
* Later that day, have someone offer him his favorite enchilada. If he says no, have NPC try to figure out why. If he eats it, have English see him, be disappointed that he's not into Tormundrism like she is.
* After a fight, English faints. She's gaunt. Hasn't eaten in days.
* more pressure, etc.
* Unless PC intervenes, English becomes super hardcore, nearly starving, and, at the moment of greatest tension, either starves to death or is somehow vindicated—becoming instantly grossly corpulent and euphoric maybe for days until she shrinks back to normal with an extra-extra spring in her step and twinkle in her eye, or maybe being able to turn the stones around her into bread. Or miserable. Or maybe just happier or better-adjusted.
3 [noun] [participle]
5 [adjective] noun
6 [personal name]ism/ianity
1 a familiar NPC is reading some of their weird literature (written backward / on strange material / in all caps or with only so 10 syllables per line / overwritten thrice at particular angles), says here man, read this, really interesting stuff. Leaves you with it.
2 a hopeless eyed man hands PC said literature, says something cryptic, won't stop hugging you if you linger
3 familiar NPC says hey your heard about [religion]? I heard they [something wildly untrue or absolutely precise], wanna go with me to a service?
4 street preacher vomits atop crate, carries on like nothing happened, has change of clothes for just such an occasion, changes on spot, after a turn longer, crowd wordlessly disperses/riots
Make It Real
1 an NPC they care about adopts one of its behaviors
2 offer player metagame reward (XP, +1 tokens, reroll tokens) for adopting and its behaviors
Show Its Behaviors (customize to make them suit you and your group)
2 harm/destruction of "redundant" or "duplicative" organs
3 charity to the point of impoverishment
4 abandonment of marriage, parenthood, filial duty
5 sweating in public is taboo
6 public nudity
7 free love
8 lying literally constantly about literally everything
9 abandonment of inconvenient infants/children (i.e., all of them)
10 abandonment of the infirm
11 absolute silence among outsiders
12 refusal to make anything excellent
14 concerted thievery to benefit a particular outsider group
15 assassinations/lynchings/beatings of particular group
16 extreme dietary restrictions; must therefore eat separately
17 cannot say certain words / express certain thoughts/feelings
18 treats members of particular group far too intimately
19 takes absurd risks (not packing water for journeys / not treating wounds)
20 suicide as moral duty under certain circumstances
Explain Its Behaviors
Just make up weird stuff. Make some of it consistent. To get the real story, track down the holy texts or magisterial body. But they may speak in code or lie as matter of principle/convenience
1 more people the PCs care about adopt its behaviors
2 offer greater rewards for PC adoption of behaviors
3 have (non)adherents act out / make demands to modify behavior
4 have behaviors become more extreme
5 reveal terrible/wonderful things about the religion
6 have the behaviors produce wonders / horrors for the PCs and those they care about