D&D 5E Scariest Monster Ever

[Rune]

I once ran a campaign in which the only undead the party ever encountered was a single zombie. That would not stay down.

I don't think it even ever landed a blow, but that didn't stop the players from being absolutely terrified of it. They ended up shoving it in a hole and running away. From a single zombie.

 

[Jan van Leyden]

Nobody mentioned the old hit yet?

Green Slime on 1st level characters. Do you prefer to die from the slime or from the cure?

 

[Agamon]

It seems to me that every time our group has had the worst time of it is when one of their number turns on them during an encounter due some sort of domination effect. This happened a few times back in our d20 days when the high hp/AC, low Will save fighter types would get charmed, which is the horrible combo of losing the meat shield and gaining a tough enemy that nobody wants to kill....

 

[Rune]

It seems to me that every time our group has had the worst time of it is when one of their number turns on them during an encounter due some sort of domination effect. This happened a few times back in our d20 days when the high hp/AC, low Will save fighter types would get charmed, which is the horrible combo of losing the meat shield and gaining a tough enemy that nobody wants to kill....

That's why the fighter in my 3.5 game took 1 level of wizard: protection from evil.

 

[Shadowdweller00]

Undead are amongst the worst for me. But the main type of scary for me is something that is both powerful and telegraphed with suitably vivid description BEFORE the PCs actually fight it. Also: weapon-eating oozes. My shinies!!!!!!

 

[BlueBlackRed]

You scare the PCs by going after their toys.

And the original rust monster could destroy so many toys.

 

[Rhenny]

A long time ago, when I was a teen, our DM invented these unseen creatures called Nerve Runners. They traveled through the sand of the desert, undetected. When an NPC was lit up by these Devils, we all freaked out and tried to find stone walls/ruins we could stand on. As players, we even picked our feet off the floor and kept them on our chairs. (Many years later, Doctor Who had an episode called "Silence in the Library" with shadow creatures that did nearly the same thing to people that touched shadow or walked in the darkness-very creepy)

 

[Shiroiken]

Oh, this should have been one of the categories.

In 1E, I ran one encounter where a Shadow (?) drained one character a level. Back then, even though we played every week, it had taken over six months to gain one level. The other players were terrified and ran.

Yea, I'm a grognard too
We got up to the higher levels, and when it takes you almost a YEAR real-time to gain that precious level... you don't fight undead. You run.

 

[Sword of Spirit]

My PC turning into a suped-up Ravenloft werewolf (we weren't sure if he was infected until it happened), killing our NPC friend, and almost dying himself.

 

[defendi]

Scariest encounters are the ones where the tone clashes with the events. For instance, one of the best encounters I ran in a modern setting had a killer hunting the party down one by one in an idyllic setting while loudly singing The Battle Hymn of the Republic. This also works when things suddenly turn on their ear. That's why little girls are so scary in horror movies. They are perverting expectations.

Next to that a foe you can't see and don't understand is pretty scary. A sniper raining death, for example.