It went pretty rough for the PC's. We lost two characters. The party was all level 4.
I had made a random encounter table and made a list of combat encounters. Most of them were relevant but I added a Shambling Mound just to spice the table up. I wasn't particularly fond of the encounter as I was scared it would just be a solo brute beatdown on the Mound's side but I gave it a 5% proc rate on a 15% chance to proc a random encounter every hour. I basically wasn't expecting it to show up for the adventure, and especially not as the first random encounter.
They ran into it at the worst possible location, too. They were near a mist-covered lake and I gave them the option to quickly hop across the stones or go around. Going around would takes 2 hours (very large lake, ooblong though) but they chose to do that. They proc'd the Shambling Mound on the last hour. I introduced it as just a plain mound of plant mass and the monk went to inspect it.
Now, I play monsters with their goal in mind. And the mound's only goal is nourishment. It can only ever eat one enemy at a time and if it consumes one, it's good for a while. Basically, it needn't continue the fight once it engulfs one member. It happened to be near a lake and it has a swim speed. It now had an escape route that I didn't anticipate when making the encounter. That's unfortunate for them, because now it has a really good strategy.
First round, it attempts to slam the monk twice. Both miss, luckily, but the players severely underestimated it. The monk spends his turn "watering the plant" thinking that it isn't a big deal. The Ranger then lights a torch because she thought the plant would be hurt by fire...it is resistant. The wizard casts Dragon's Breath on his familiar and uses Fire as the damage because he also thought the plant would be hurt by fire...it is resistant. The moon druid casts"Ice Knife" because he thought since fire wasn't working, the plant would be hurt by cold...it is resistant. Well, great, not alot of damage and the monk is playing around with it.
The Mound recognizes the ranger as a bigger threat as it was close and the monk had yet to do damage. It makes two slam attacks, both hit and one crit. The Ranger is now unconscious and engulfed. They make the first save, fail the con save meaning a death fail and the monster slams them (with advantage) while she's inside the Mound's body. That outright kills her. He shambles to the lake and begin to swim. The Monk and Druid (now in wildshape which is important) are chasing after. The monk uses his spear to attack and the druid is a direwolf biting. The Mound realizes the monk is chasing it in an area where the mound has the advantage and changes targets. He lets go of the Ranger's corpse and slam attacks the monk twice. Both hit and one crit...again. The monk is now unconscious and engulfed.
Here's the tradgedy: the druid attempts to healing word the monk to give him a fighting chance. I remind him he must revert back to his original form. Then, this is really depressing, I remind him that reverting takes his BA and he can't cast healing word. He never took cure wounds because he never expected to need it more than healing word. The druid instead grabs the Ranger's lifeless corpse and swims back to the surface, leaving the monk to be the Mound's dinner.
The fight was a "Hard" encounter and they only fought 2 medium encounters before with a short rest in-between. I wasn't expecting anyone to die, much less 2 party members. They did roll poorly during the fight and I rolled a crit at least once every round. They also underestimated it. It was some sad deaths but now they're bringing more characters to join whenever they have the opportunity.
This sucks for the survivors too because they are obviously down for manpower and now the ranger, the ticket for them to not get lost and not be in difficult terrain, has now died. They're close to the settlement they're trying to reach but they now run the risk of getting lost and taking longer.