PC's were after a magic item lost in a deep jungle haunted by an evil spirit. Before leaving town, they research the spirit and learn that it is associated with both plantlife, and with unseasonable rainstorms.
After exploring the jungle and dealing with the local creatures (including some really nasty carnivorous plants) for two days, they finally, through clever uses of divination magic and puzzle solving, managed to find an entrance to the cave system that the artifact is hidden in.
This cave system is infested with shambling mounds, which are even worse than the plants that they fought on the surface. The party warlock recognizes that the shambling mounds are whispering in an infernal dialect as they attack, really creepy and confusing phrases about "cracking open the hard shells to drink the sweet water within" and "so long since the water of life has fallen; these ones drip their own." The shambling mounds are really stealthy, they attack from the darkness or from under pools of water, and their leader is a shaman who can cause the roots that emerge from the cave walls to grab the PC's, or hypnotize them into walking into danger. He makes hit and run attacks throughout the dungeon crawl.
Finally, the PC's reach a central chamber. There's an underground pond of black water with a horrible shrine emerging from the far side of it; the roots that grow from the cave ceiling and floor have been formed into rows of giant teeth, and gems set into the wall above it to serve as a trio of eyes. Inside the "mouth" of the demonic face are the ancient skeletons of the original bearers of the magic item, surrounded with vines and roots, placed there as offerings by the shambling mounds. The shambling mound shaman makes his final stand here, but when the fight goes against him he retreats into the water and lets his minions hold the PC's back while he casts some complicated spell. As he casts it, the roots and plants throughout the entire cave twitch, and there's a feeling of electricity in the air. With some successful Arcana and Religion checks, the PC's learn that he's casting some kind of communion spell.
PC's kill the shambling mounds and take the magic item and other treasure, but they're very low on hp and spells. With their loot, they finally climb out of the cave and into the jungle...but find that the sun is no longer shining. The sky has become overcast. Rain begins to fall, and quickly gets harder. And then I start playing
this and telling the PC's that they feel very strongly that they should run.
The forest has changed while they were underground. The trees and brambles have reformed themselves into obstacles. Plant monsters leap out of nowhere to attack and slow them down. Meanwhile, the rain comes down harder and harder, and they start to hear a deep, demonic voice chanting from the trees behind them, lulling them into slowing down, turning back, giving in (will saves to resist). They just barely managed to get out of the cursed valley before the demon could catch up with them (literally, they had one round left). One of them was actually about to turn back to buy the others time. I didn't stop looping the Spooky's House of Jumpscares music until they were out of the valley.
All four players had eyes the shape of saucers by the end. Two of them walked home together because they were afraid to go alone.
UPDATE: at least two of the players are furious at the demon for scaring the shit out of them, and are determined to come back in a few levels and kill the fucking thing. I've been thinking about letting them do that, and I've come up with an idea.
The rainstorms that surround the demon aren't just for decoration; they are actually the source of his power. When his rain soaks into the soil and into the roots of the jungle plants, they bring the forest under his dark influence, which is what allows him to manipulate the terrain, animate plant monsters, sneak through the forest swiftly and quietly despite his bulk, etc.
The PC's are in the process of learning this via research of old tomes and consulting summoned spirits (one of the PC's is a warlock with a particular knack for divination). Once they're done with that, they'll probably figure out that coming back to the jungle once they're high enough level to cast Control Weather is the key to victory.