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I know that monsters often have treasure, and that if a monster has say a +1 magic longsword as part of the hoard described in the treasure entry he will likely use it, and this is figured (somewhat) into the CR.

But what if as a DM I grant a monster an item well out of its price range for use against the PCs, but which they can't take as treasure? Is there any way to know how much this item would effect the CR of the monster or the overall encounter level?

minnmass
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Critical Crafting
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    Similar question. – Hey I Can Chan Nov 07 '17 at 17:05
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    I think this question might be more readily answered—and maybe even more useful to you—were it more specific. I mean, a goblin chugging a potion of fly before its encounter with a four level 1 PCs is a lot different than equipping an adversarial level 5 paladin with the shield of the sun that's ensorcelled to return to her temple upon her defeat when she's due to take on a group of level 3 PCs! In short, what's the actual situation? – Hey I Can Chan Nov 07 '17 at 18:14

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Slight frame-change: don't worry about the price.

It sounds like you're creating a custom variant of a monster, in effect: you've got a bugbear with a Vorpal blade or a wizard with goggles of True Seeing or an Invisible Stalker with a bracelet of Silence. In any case, the item is "soul-bound" to the wielder and is destroyed upon its death. That sounds a lot like slapping a poisonous stinger onto a dragon's tail slap.

The specifics will vary, but this GM's advice is to look at the Adding Special Abilities section of improving monsters:

You can add any sort of spell-like, supernatural, or extraordinary ability to a creature. As with a class level, you should determine how much, or how little, this ability adds to the creature’s existing repertoire. A suite of abilities that work together should be treated as a single modifier for this purpose. If the ability (or combination of abilities) significantly increases the monster’s combat effectiveness, increase its CR by 2. Minor abilities increase the creature’s CR by 1, and truly trivial abilities may not increase CR at all. If the special abilities a monster gains are not tied to a class or Hit Die increase, this CR increase stacks.

A significant special attack is one that stands a good chance of incapacitating or crippling a character in one round. A significant special quality is one that seriously diminishes the monster’s vulnerability to common attacks. Do not add this factor twice if a monster has both special attacks and special qualities.

Make sure to "scale" your evaluation of these abilities by the monster’s current CR.

Treat the effect of using the item as an SP, SU, or EX ability (keep in mind the fact that SP and SU are vulnerable to anti-magic effects, like magic items are).

That bugbear's so good with their sword that, as an EX, they will decapitate their victim on a crit (like the Razor Boar's tusks).

The wizard has made a pact with a demon which has granted them the supernatural ability to sense the presence of any living creature within line of effect, piercing all illusions.

The Invisible Stalker's been infused by the demielemental plane of silence ('cause, why not), and sound within 30 feet is immediately absorbed.

Which, this GM would peg all three of those abilities as "significantly increasing the monster's combat effectiveness".

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minnmass
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  • By frame change do you mean frame challenge? (If so, I would recommend not using that terminology since nobody seems to use it except us) – doppelgreener Nov 07 '17 at 18:19
  • Bouncing off of this, what would you consider adding something like fast healing/ 5 or DR 3/bludgeoning? Are these technically natural abilities? Supernatural? Would you amend the CR the same way? – Critical Crafting Dec 07 '17 at 19:47
  • @Khaldhool: that sounds like an excellent new question ("how do I determine the CR adjustment of modifications to existing creatives?"). Briefly, both examples could be EX, SU, or SP (though, SP is the least likely). The in-game reason for the effect matters (DR from thick hide = EX vs. DR from magical protections = SU or SP). I would probably amend CR the same for any, though would "round down" for magical effects due to anti-magic. Anything that takes an action is cheaper than the same effect without taking an action (eg., SP "heal 5 HP at will" is cheaper than "Fast Healing 5"). – minnmass Dec 07 '17 at 20:43