What makes a monster terrifying?

[Lakesidefantasy]

There meazel is pretty terrifying if it can teleport one of us off to a dark location where it can strangle us to death.

 

[Tony Vargas]

Stop me if you’ve heard this one before:
“The monster attacks... Ragnar, because he did the most damage to it last round.”

Sure, or "...because he looks the toughest." It's sorta the old-school gentleman's aggro/mark, yeah.

It’s a very, very common “tactic” used by DMs, usually to avoid seeming like they’re unfairly targeting anyone. And it never fails to kill the tension in an encounter. It becomes immediately clear that the monsters are going to behave in what ever way will seem the most fair to the players, instead of making tactically prudent decisions. Simply playing the monsters like they want to win goes a long way to making them scary.

Well, it'll make those monsters more challenging.
And it'll make the PCs nominally there to 'protect' their allies look pretty bad.

 

[Lakesidefantasy]

Also, when monsters say weird, nonsensical things. We were actually terrified when we were lured into a swamp be a hag uttering the word "hello?"

After we defeated her and were catching our breath, we began to hear multiple calls if "hello?" Issuing from the mist. Needless to say, we didn't stick around to fight more hags.

 

[Lakesidefantasy]

I love gibbering mouthers, and I always like to have them gibber weird things. One wandered around saying "what, what, what is it?" over and over and over and over and over... well, you get it.

 

[Tony Vargas]

I love gibbering mouthers, and I always like to have them gibber weird things.

"kumquat!"

 

[Charlaquin]

Sure, or "...because he looks the toughest." It's sorta the old-school gentleman's aggro/mark, yeah.

Well, it'll make those monsters more challenging.
And it'll make the PCs nominally there to 'protect' their allies look pretty bad.

Yeah, 5e desperately needs more “aggro” mechanics to make tanking a viable tactic. Marking was one of those great ideas from 4e that got tossed out.

 

[darjr]

Strong grappling gargoyles on a cliff face terrified my players when they were clinging to a cliff 1000 feet up and no one had feather fall.

but to be fair none of them new it was there until the sea of crawling undead gave them no choice but to crawl down, while it was raining undead. I guess that was already pretty scary.

 

[Tony Vargas]

Y'know, the old capture scenario, so commonplace in fiction, seems to really freak players out.

 

[BookBarbarian]

The most terrifying thing about any monster is uncertainty. Monsters represent an uncertain possibility for the future, where things are not going as well for you as you might hope.

The two major factors are: 1) how bad things could go; and 2) how likely they are to go that badly.

I think you hit the nail on the head here.

And I think players decide "how bad things could go" based on what they fear the most. And I think what they often fear the most is losing something they've gained or losing control of what they can do in that moment.

The former we see all the time have a PC's boots stolen, much less when they lose a powerful magic item or macguffin.

The latter is what I think fuels the players dread that first time they see a troll regenerate, or a werewolf shake of that attack. The realization that what they tried is useless and they don't have control over the situation like they thought the did.

Overall, I think we fear feeling helpless.

 

[Odysseus]

a really big mini usually works