I have a low level party coming up on a boss fight and for he style of the campaign there is a certain demon that works really well as the boss. The problem is the CR for this creature is far too high for the party. Is there a template or rule system for decreasing monster power much like monster advancement rules but reversed?
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\$\begingroup\$ Should answers limit themselves to official material only, or is third-party material acceptable? \$\endgroup\$– Hey I Can ChanCommented May 29, 2017 at 20:39
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\$\begingroup\$ I would prefer official only but if there isn't anything official, 3rd party works too \$\endgroup\$– Critical CraftingCommented May 29, 2017 at 21:15
3 Answers
There are a couple of negative CR templates (e.g. incarnate construct), but none of them apply to demons to my knowledge.
Ultimately, even if such rules existed, they wouldn’t work well, which is probably why they don’t exist. The issue is that while you can advance a creature (at least a couple levels) by just increasing numbers, and you can always staple more abilities on (e.g. with templates), simply decreasing numbers will not always work for de-powering creatures because they have special, non-numerical abilities that are problems for lower-level adventurers. The only reason stuff like incarnate construct kind of works is because it strips a construct of all the myriad excellent construct traits, i.e. the template knows of some abilities it can remove to reduce CR (and even so, it’s still eminently abusable).
So really you have to reduce CR manually, on a case-by-case basis. We can probably help with that, you know, if you want to ask another question about your particular case.
Negative Levels
While not a template, applying an arbitrary number of negative levels and calling it a day is a quick and dirty method of reducing the effective CR and ECL of a creature.
For example: grant 2-4 negative levels to a given creature and call that a template, "Wounded (X)" or perhaps "Weakened (X)", where (X) is the creature in question.
However, as CR and ECL are NOT the same thing, how to recalculate the CR based on negative levels is a bit of a mystery not answered in the rules. Also, please keep in mind that all CRs (and ECLs) are estimates, and can be influenced by terrain, weather, allies, and strategic planning on the part of the creature/monster.
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\$\begingroup\$ That's pretty clever, especially since the negative levels will also reduce the effectiveness of many creature's the special abilities. To put some numbers behind this, is it worth suggesting reducing a creature's CR via negative levels be used in the inverse fashion that adding Hit Dice increases a creature's CR? \$\endgroup\$ Commented May 30, 2017 at 19:15
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\$\begingroup\$ That was more or less the gist of my thought. Simply removing HD is also a possible option, though that gets a little weird if you go outside the standard HD range for the creature - and not all abilities are affected by HD changes, which can also make the subsequent CR calculation difficult. \$\endgroup\$– nijinekoCommented May 30, 2017 at 19:44
Sure, but it's a bad idea
...if taken too far.
You can level a monster down by HD, just like you advance it. I've done it and it works okay. The problem is that many of the monster abilities turn it into a glass cannon: its abilities are much more likely to kill the PCs, even though HP drop enough that they can kill it quickly too.
(Special mention goes to a de-leveled purple worm — Swallow Whole stays very nasty at lower levels.)
If you have an ideal "boss" in mind, and it is more than one or two CR too high, instead either try building a new monster along similar lines at the correct CR or re-fluffing a correct-CR monster to match your ideal (re-fluffing is a versatile and good solution to many problems).