toucanbuzz
No rule is inviolate
If we're mixing "horror" and "terror" together with the idea that PCs in 5E aren't really "scared" anymore of monsters, there's a rationale for that. In AD&D, there were monsters that DID terrify players:
If I had to make monsters invoke terror, I'd have to go back to making them have permanent consequences or extremely nasty to deal with, not just resistant to stuff but completely immune.
- Vampires and other life-draining undead. You lost levels. 2 per hit for a vampire. You didn't get those back after a good night's rest. Gone forever unless you had access to extremely high level magic, and even then, you were only restored to the starting XP for that level. It was terrorizing to face undead. One hit and you're screwed.
- Illithids (mind flayers). They had a 90% chance to shrug off any spell and could instant-kill you in a round or two with brain-eating attacks, of which they had 4. When they combined a vampire with mind flayer (up to 8 levels lost per attack round), yeah. The baddest worst monster to ever run into.
If I had to make monsters invoke terror, I'd have to go back to making them have permanent consequences or extremely nasty to deal with, not just resistant to stuff but completely immune.