subtle ways to include foreshadowing
one character knowing something offhandedly that they shouldn't, isn't addressed until later
the crow rhyme
colours!! esp if like, blue is evil in your world and the mc's best friend is always noted to wear blue...betrayal?
write with the ending in mind
use patterns from tragic past events to warn of the future
keep the characters distracted! run it in the background until the grand reveal
WEATHER.
do some research into Chekhov's gun
mention something that the mc dismisses over and over
KEEP TRACK OF WHAT YOU PUT. don't leave things hanging.
unreliable characters giving information that turn out to be true
flowers and names with meanings
anything with meanings actually
metaphors. if one character describes another as "a real demon" and the other turns out to be the bad guy, you're kind of like...ohhh yeahhh
anyways add anything else in the tags