1. Star Haunt. In the mood for a ghost-like tale? Send your snooping PCs to the Celadon Forest, and enter the star-shaped castle ruins called Star Haunt. The walls of this ruin glow in the dark due to peculiar moss, and inside the creatures that lurk have an innate displacement effect. Anything, including perhaps the original Oeridian residents slowly turn into incorporeal undead. Star Haunt is a nice haunted house style romp, and its definitely not a place the heroes will want to stay in for long. Check out this ruined castle in the 2E sourcebook, Marklands.
2. The Blood Obelisk. Fading Lands are weird demiplanes that overlap the Flanaess at set times then vanish. Usually these are strange lands like a faerie realm, but one, the Blood Obelisk of Aerdy is quite frightening. The fading land is a black plain of dust and rocks. Here, unwary heroes will run into metal-skinned trolls, iron golems and other metal monstrosities. At the center of the plain is an unholy magic obelisk dedicated to Hextor. What is its purpose and how one gets here is up to each DM, but if you'd like to know more, check out From the Ashes.
3. Wormcrawl Fissure. So you think zombies are too easy in D&D? Then send your players on a quest to the Wormcrawl Fissure! This ravine is on the western end of the mighty Rift Canyon, and is best known for being the home to the vile creator of undead, Kyuss. Bandits and servants of Iuz avoid this place if possible because the zombies here are even more than your run of the mill Sons of Kyuss seen in Fiend Folio. They are stronger, tougher, and have other nasty powers. Read more about this eerie fissure in Iuz the Evil or the Age of Worms adventure path, particularly Dungeon #134.
4. Howling Chasm. In the mood for a mind-twisting mystery? Go to the Howling Chasm in the Tilvanot Peninsula. This chasm is in the south face of the Okalasna Plateau, domain of the Scarlet Brotherhood. If an aspiring explorer can get to this remote land, what they will discover here can be a chilling mystery. Those who enter seemingly return years later to learn less time has passed on Oerth. What's worse is the maddening visions and memories they are left with after coming back to their own time, or the unexplained alterations to their body. The Chasm could be literally anything a DM desires to spook and confuse their players. Naturally, you can find this spooky locale in the 2E sourcebook, The Scarlet Brotherhood.
5. The Sanitorium. The city of Molag in the Horned Society is bad enough as evil locations go, but within the Hierarch's Palace is a site that is downright horrific in its own right. This imposing edifice is the most well-known location in Molag, infamous for being ran by priests of Nerull. Here at the Sanitorium, people near death are captured by the priesthood, and brought to be used for their twisted necrotic interests. This scary building is an excellent location to host a thrilling rescue. Will the players offend the Reaper, or will they be his next victim? Check out this location, including the Molag city map in Iuz the Evil.
7. Screaming Valley. So the characters are passing through the Cairn Hills? Want an alternative spooky site to the much maligned Doomgrinder? Try passing through the Screaming Valley. This is a bleak, four-mile long stretch of land where no insects or birds dwell. Half-way through this barren place, one can find a mass of sandstone riddled with holes that create a loud noise for miles as wind rushes through the valley. This unnerving valley is home to an eclectic selection of undead and creatures. DMs who want to subject their players to this chilling valley can find it in the From the Ashes boxed set.