Necrotic Shroud
Here’s another entry in my easy encounters series, for an explanation of why this is so trivial to succeed at, see Short Encounters to make players excel.
Ravenous Beetles
Target Ability: Aasimar’s Necrotic Shroud racial ability, or anything that produces fear
Ability Summary: Eyes and Flightless wings transform you into a fear provoking terror. Non-ally creatures must succeed on a Cha saving throw (DC 8 + Cha mod and Prof bonus) or be Frightened
Description: For some time now, the bioluminescent fungus that provides enough ambient light to navigate through this portion of the underdark has been showing signs of stress. Large patches of fungus have been stripped bare, and what remains is stressed.
The source of this stress quickly becomes apparent. A cacophonous chittering noise builds as you approach a large bowl shaped cavern. Lining the floor and some of the walls is the source of the noise, and damage.
Fist sized beetles, millions of them, carpet the cavern, munching on whatever they come across. Their black, shiny carapaces reflect light, and the ground looks like a roiling carpet of stars.
Individually these beetles are not a threat. They can deliver a nasty bite, but nothing worthy of a hp loss. Collectively though, they could be deadly. To reinforce this, you could describe some large skeletons being stripped of flesh by the beetles.
If they are attacked they behave aggressively, but are essentially cowards. Demonstrate this by having them shy away from agressive movements toward the swarm. They automatically fail any save verses a fear effect, and will shy away, leaving a path for the adventurers. The beetles can be intimidated, but this is a slow exercise. If attacked, or if the swarm is entered, nearby beetles will crawl all over the PCs and bite, automatically hitting and dealing 2hp a round. Make it clear to the PCs that they can retreat, but to fight their way through the swarm could take hours.
Again, the idea here is to allow a character who can produce fear to trivially bypass this encounter. This one is a bit less obvious than previous ones, the DM really needs to lean in on describing their cowardice, while maintaining the threat of the swarm.
For more of these, check out the encounters tag, which I just now have noticed is not actually shown on this page anywhere. I’ll add it soonish, but in the meantime, here’s a link: Encounters