It could be a throwaway comment, a weird dream, a madman babbling dire warnings that don’t make sense until later.
No matter what form it comes in, really good foreshadowing will give you a sense of unease. Really good foreshadowing will seem obvious in hindsight. Really good foreshadowing will force you to set the book down for a moment while you collect yourself because OH GOD SOMETHING’S COMING.
So, with that in mind, here are some lines of literary foreshadowing so brilliant you can practically hear Han Solo saying, “I’ve got a bad feeling about this.”