Cadfan said:
I design a monster that has an ability which inflicts on an opponent a DC 20 fortitude save. Failure kills the target. Its a magical effect that has no other roll to hit, and occurs at range.
1) At what level is this monster an appropriate encounter for a character? If you don't want to answer with a particular level, just answer with the fort save you think a character should have at the point where this monster is an appropriate encounter. Please specify whether the fortitude save you've given is the expected low fortitude save for the group, or the expected high fortitude save.
There isn't enough information to answer this question in your post. But, let us assume for the sake of argument, that this monster is otherwise statistically identical to a goblin. Is that fair? You haven't specified how the ability works, either, which would be necessary to answer the question. Let's imagine that it was a 1/day spell (edit: targetting 1 individual), and then imagine that it was a gaze attack, okay?
As a spell, we are creating a sort of one-shot wonder. It uses the ability, you either save or don't save, and then it is killed (regardless of PC level). We are looking, then, at what chance we want to have as to a single PC death. If your group travels with cohorts or hirelings, we might be talking about a single "red shirt" death. Depending upon the group, then, this could be used in various ways from 1st level to about 6th.
1st level PCs generally encounter the creature in a set-up where it shows its power against someone/something else. They then slaughter it (and get XP for a goblin). This now sets up tension for when they see the creature again. By the end of 6th level, using the 3e power curve, they might no longer feel any tension from the creature. I would personally change the DC to about 15, making it more of the 1st-4th level range, depending upon how the encounter is set up and how the group handles problems.
CR is, as I've said, and as WotC has confirmed, always something of a guess. Nonetheless, I would venture to place this creature at CR 3 if it used its ability against the PCs, CR as goblin otherwise.
If this was a gaze attack, this might be the means to create an encounter where the PCs are encouraged to close their eyes and fight. Again, this would be 1st-4th level in range, though one might again lower the DC of the save. Obviously, the save DC would be perfectly fine if you upped the other traits of the monster. It is also perfectly fine if there is ample warning; i.e., if the PCs are deciding between (1) not having the encounter, (2) fighting blind, and (3) risking the gaze attack.
This creature would fall under the "Special: Easy if Handled Properly" category. I would make the creature CR 2.
IMHO, of course. And off the top of my head.
RC