OK, so this has been a minute. But believe it or not I have been ruminating on this, although it took me some time to try to wrap my head around the math for it. And the result I've come up with is actually really interesting. Note: Again, I'm undertaking this exercise in an attempt to put an m20 ruleset together that's compatible with 5E materials. If you don't care about being compatible, then this post is nothing but a fun (?) mathematical proof of sorts.
First, my logic, which I'll try to lay out here in a way that's somewhat sane:
- CR is a measure of difficulty/danger of a particular challenge (whether creature or other situation, e.g. trap).
- It also directly (per the 5E DMG) determines the reward PCs get for overcoming it.
- That reward appears to be increase over time, despite the fact that the actual level of challenge (i.e. "a CR level of X indicates it should be difficult, but not deadly, to a party of four characters of Level X) doesn't.
- It follows then that the number of CR needed to rise to a particular level would equal the number of XP (again, per the 5E core rulebooks) a PC needed to reach that level versus what was needed for the previous level, i.e. the "delta" between the current and new levels, divided by the number of XP each CR at that particular level represents, multiplied by four.
For example, in 5E, going from level 7 (23,000 XP) to level 8 (34,000 XP) requires a "delta" of 11,000 XP. A CR 7 creature awards a total of 2,900 XP, which works out to 414 XP per CR at level 7. To calculate the number of CR needed to rise from level 7 to 8 in this proposed system:
(XP Level 7 Delta / XP per CR at CR7) * 4, or
(11,000 / 414) * 4 = 108
So using a fun application called a spreadsheet, I calculated the total EL needed to reach each level up to 20, and compared it to the number of CR required for each level using the logic outlined in point #4 above. This is the result:
Though these methods diverge a bit through the upper-middle levels, overall they match up pretty well. Meaning, assuming a couple of other mechanics, you could simply pick up any creature from a 5E source, and award the PCs a number of "EL" equal to the creature's CR, and use the progress as outlined in Purest Essence for advancement (10 EL * current level to reach your next level).
Some of these assumed mechanics, as I see them, are as follows:
- You're using 5E-style HP progression, otherwise creatures/challenges are bound to get increasingly lethal if PCs only receive 1d6 per level.
- PCs should have feats or some mechanism that buffs their damage as they progress, otherwise creatures will become increasingly difficult to kill. Now, given that the entire Feats concept is optional in 5E I'm not sure how much water this one holds, but I also haven't heard the results of any campaign that's chosen not to use them in terms of how much deadlier it is or isn't.
- CR for each and every creatures is awarded to and divided evenly among the PCs. Again, the "add 1 EL for each doubling of enemies" makes absolutely zero sense to me given how much deadlier encounters become when you add enemies. Even less so when I'm trying to make a direct correlation between CR/EL and XP, which traditionally is divided evenly.
All this said, I'm choosing to believe that the accuracy of the line graph above is not just a coincidence of math, and so I'm pretty happy with the concept of using CR interchangeably with EL.
If anyone cares about this topic and also manages to understand the mad ravings above, I'd love some help in looking for faults in this logic.
If it's indeed sound, I'd want to do next is an m20-style distillation of the rules on calculating CR/EL for a creature/challenge given its stats, and vice versa, guidelines for creating a creature/challenge of a particular CR/EL from scratch. This should take into account not just Hit Dice, but attack, defense, and other capabilities as well. But also not require a PhD to use. Again, while the HD = EL standard is probably pretty accurate, it's a little simplistic for my taste. I'd like to more accurately account for things like high damage-output creatures (e.g. "normal" HD/HP but big attacks) and extra-tough creatures ("normal" attacks, but higher HD/HP and or AC).