D&D 5E 5E Maths

[surfarcher]

How do you quantify things like the Intellect Devourer, though? Technically it could kill a 20th-level character in two rounds and yet is only a CR 2. It doesn't do it through damage, it has low hit points and a few resistances, but otherwise just a few of them could TPK a group without the PCs ever even having a chance (other than the saves) to fight them (they can do it from hiding and teleport in, never revealing themselves).

With oddball creatures like that it gets tricky, for sure and a nasty DM can do some terrible things with such creatures. CR seems to have it's biggest emphasis on Damage, then HP with AC and Attack bonus being fine-tuners. The Intellect Devourer has 21HP and that clearly labels it as a apprentice level creature and the lower end of apprentice at that. It also has defences appropriate to low apprentice level.

It's pretty rare to see 21HP outside of CR1/2 or CR1 so...