Nightmare Fuel / John Carter of Mars

• Valley Dor. At first, it appears to be the closest thing to a paradise on Mars as lush and fertile land in contrast to the arid wasteland that is the rest of the planet. Once you reach it, its revealed the valley is also inhabited by white apes and plant men, some of the most fierce predators in Barsoom. If you managed to survive them, you will be captured and enslaved by the White Martians if not devoured by them. While horrific, it's said those that die are the lucky ones. Thuvia, Princess of Ptarth, was their Sex Slave for 15 years.
• The ending to Gods of Mars: Carter briefly reunites with his wife Dejah who has been captured by the First-Born after being separated for 10 years, only for her to be trapped inside a Temple with no way out for an entire year without enough food to survive. That on itself is bad enough but the last thing he sees before the door closed permanently was a very jealous Phaidor pulling a knife and trying to kill her. This was so traumatizing that Carter hovered over the Despair Event Horizon.
• The Carrion Caves in the third book The Warlord of Mars. A putrid graveyard riddled with the corpses of Green men and apts, it's deemed a cursed place by the Barsoomians and no one dares cross it. It's how the Yellow Martians have kept themselves hidden for so long, since that is the only possible entrance to their domain on foot, while their advanced technology keeps any flying ships of getting too close.
• Female White Martians are the primary targets of the First Born pirates whenever they raid Valley Dor. Because they never take any males alive, it's believed the females suffer A Fate Worse Than Death and to make matters worse, the men are forced to abandon them to their fates out of fear of retribution. It's hard to not feel bad about Phaidor when she pleads Carter to save her when she is dragged off by them.
• The work of the brilliant but amoral Ras Thavas takes a nightmarish turn in Synthetic Men of Mars. As the title suggests, he's gone beyond his already-macabre work in transferring brains between bodies and started producing artificial people. The synthetics themselves are generally deformed and malicious, but that's not the worst part. "Something" has gone wrong with one of the vats, and now it's not producing synthetic men at all - it's just a screaming, seething, ever-growing and constantly regenerating mass of arms and faces and organs. In the end, the Helium air fleet have to Kill It with Fire.