This is based on opinion. Please don't list it on a work's trope example list.
Nightmare Fuel / Fate of the Jedi
Sinkhole Station. Located between two black holes, it's a ghost station full of dead bodies, failing systems, and a cult of beings that are happily dying slowly so that they can become 'one with the Force'.
Force Psychosis. If Ben's nightmare involving him going insane because of it is true, this will only get worse.
Never step into the Lake of Apparitions unless you want to end your life in the Depths of Eternity.
Drinking from or getting submerged in the Pool of Knowledge will turn you into an Eldritch Abomination.
The drochs, bugs that cause the Death Seed plague. They burrow into your flesh and slowly eat you alive from within, and once they infect you, they're completely undetectable.
Raynar Thul's return to the Killiks. This is essentially his nightmare, as everything that was Raynar Thul was subsumed by the Killiks into Unu Thul. Given how long it took him to recover from his last stay with the Killiks, this is probably one of his greatest fears. Sure, Luke sends a group of Jedi after him not long after, but every moment in their link is probably the worst thing that could happen to Raynar.