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Yeah, anything which bypasses the 'meat' of hit points and starts debilitating characters directly can be pretty scary. However, things which actually take a character out of play completely can have the reverse effect - nobody looks back fondly upon the awesome fight they watched happening between the other players and the DM while their character spent the whole battle paralysed.For me? When I'm playing, it usually revolves around Non-HP consequences:
1) Petrification, Poisoning, Paralyzation, etc.
3) Loss of equipment (Disenchanter, Rust Monster, etc.)
That sort of thing.
What's "terrifying" in the context of a TTRPG, where you're all sitting safely around a table?What makes a monster terrifying?
So I've now DM'd a lot of people that have started out as complete newbies to D&D and have progressed to veterans. And the one thing I have consistently noticed is that, relatively quickly, they have grokked the salient driving force in 5e. Whatever small bells and whistles a monster might have, it is, in the end, a bag of hit points. Whatever resistances it might have, whatever abilities it might possess, in the end all you need to do is just cause more damage. Every monster is a nail, and DPR is the hammer. Once that salient point sinks in, the monster qua monster is no longer scary, or even that interesting. The tactics or combat might be, but never the monster itself. Perhaps there might be a save or suck here, or a nifty effect there, but it's all going to be somewhat familiar.
2. Be unfair.
I should have added Dominating/controlling a PC can be pretty terrifying. For some reason the party's barbarian attacking the rest of the group is much scarier than just some run of the mill monster.