There are numerous ways of achieving this.
However, before posting my answer I would like to emphasize the following personal opinion:
It's OK if your players miss your hints
Some comments mention that dropping hints is difficult in role-playing because most of the time the players won't get them unless you become too obvious. I totally agree with this statement, however, I believe that this is perfectly fine.
In art (books, movies) this is called foreshadowing. The point of foreshadowing is to NOT to warn the reader/viewer that something inevitable is coming, rather, foreshadowing exists so that, in retrospect, the reader/viewer finds the revelation totally acceptable. If there's no foreshadowing the reveal appears to be nonsensical or the result of bad writing.
In other words, it's ok if your players miss your subtle signals because they will appreciate them later on.
Now, to the actual answer. The best way to foreshadow a big turn of events is:
For example, the village looks like it came out of a fairy tale, yet, if the group looks closely, they will find out that some things just don't fit. For example:
- A disturbingly evil-looking knife in the house the mayor.
- A statue of their deity which doesn't look really nice.
- A series of jars containing something unspecified.
- There are no elders/children/men/women (pick one).
Why Anomalies Work: A Personal Experience
I was playing a Viking campaign a few years ago. My PC was a Cleric of the Grave Domain and his deity was Hel, the Goddess of the Underworld. My PC was following Hel's commands religiously, however, during the course of the campaign, some things were "off". For example, at some point, my cleric decided to build a Spear that would draw powers from the goddess herself. After the expected quest, I asked my GM whether the spear deals necrotic damage. He said that the spear deals poison damage. OK... I didn't pay too much attention.
Then, after speaking with a few other Clerics of Hel (my PC used to be a hermit so he had never encountered one of them before) I learned that Hel never takes a life nor gives life. To kill or save a person's life is forbidden. Death is the only sure thing after all. However, my PC had assassinated 12 people in the past and saved a few others because Hel herself instructed him to do so. Now, this was definitely weird, but, because I acted like a classic player that won't take a hint, I simply thought that these clerics are obviously frauds and my PC is the true follower of Hel.
You may have guessed where this is going but, yes, Hel wasn't Hel. It was Loki (because of course it was). My PC had spent his life worshiping Hel only to find out that he was a Cleric of Loki all along. And the hints were there but I ignored them!
Now, the important takeaway of this personal experience is that I may have missed these obvious hints but this is OK because, after the big reveal everything made sense and I couldn't be happier about it. The anomalies were super effective in an (un)intended way. Therefore, in my personal experiences, anomalies are very effective from a narrative point of view.
If someone wants to read more, I have written a blog post about this and how it was my all favorite moment as a player. In there, I describe several other things that were "off" for the rest of the PCs and how they made the big reveal even greater.
Apart from introducing anomalies there are a few other techniques. I don't have personal experience with them, however, both are classic TV tropes:
Everything is perfect. Too perfect.
Everyone in the village is extremely happy with their lives. Everyone has the perfect job, the perfect family, the perfect friends. Unhappiness is almost unheard of.
This a classic TV trope in fantasy and sci-fi. There are several episodes in Star Trek for example where the crew meets a utopian civilization only to realize that things are more sinister than what they look like.
An unknown element
If not a classic TV trope this technique is definitely being used in a multitude of fantasy and sci-fi stories. The idea is that there is a name/event/place/object that's shrouded in mystery. Examples:
- The gnomes are excited about a celebration that's coming soon,
yet, when asked, they never reveal what the celebration is about.
When the celebration begins, the group discovers that it is the annual summoning a demon.
- The gnomes speak of their great deity
but the group has never heard this name before. It is later revealed that it is another name for a known demon.
- There's a place that it's absolutely forbidden for the group to visit. It is later revealed that this is their sacrificial ground.