The PCs will soon be entering their first "Megadungeon". And I want to emphasize how dangerous this environment is - not by throwing them into meatgrinder fights, but by having them come across the remains of earlier adventurers who died horribly. And I could use some suggestions from others!
Here are the rules:
First, describe the scene - whatever the PCs are seeing.
Second, describe what actually happened in case the PCs investigate and make some successful skill rolls, use divinatory magic, and so forth. Here is an example:
1. The PCs come across a chain mail which has seen some battle damage but is largely intact, as well as the blade of a handaxe. Both are covered by a thin green-white residue. Searching further, they come across a belt buckle and a small number of coins, all of which are likewise covered in residue.
(A gelatinous cube came across the body of an adventurer, digested everything organic, and left the metal parts behind.)
A classic is extremely lifelike statues but they're a bit mangled and have horrified expressions on their faces. Upon further investigation it's not statues, it's people that fought something like a basilisk that has the ability to petrify. Drop in wherever feels right for the target creature.