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In Volo's Guide to Monsters on p. 56 in the Hag's Weird Magic chapter a mechanic is described in which a Hag can use weird objects to produce CR-appropriate spell effects.

If you want a hag to use a weird object of this sort in a combat situation, provide her with an item that produces a CR-appropriate spell effect when the hag manipulates or activates it.

The example being a CR3 hag producing a 2nd-level spell effect.

This is not further described, neither in VGtM nor in the Dungeon Master's Guide. Even the table for creating monsters has no column for appropriate spell levels.

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We can read CR-appropriate as "not changing her damage output or effective HP"

In the absence of an explicit definition, we can just read the passage ordinarily. An alternative interpretation of that passage would go something like: "If you give the hag the ability to cast extra spells, give her spells that don't modify her CR".

So what determines CR? In the DMG tables, CR is the average of the offensive and defensive CRs. The offensive CR is determined by damage per round (assuming that the spells the hag casts are at her spell DC). Therefore, any spell that does damage that's equal to or less than her average damage output would be appropriate. Conversely, any spell that does not significantly increase her effective HP/AC would also work.

For example, giving the hag Power Word Kill would significantly increase the hag's offensive CR, and thus would not be CR-appropriate. However, an item that casts Astral Projection might still be CR-appropriate, since it doesn't deal damage in combat and it doesn't increase her survivability.

This calculation kind of breaks down when you get to spells that remove or incapacitate PCs without actually dealing damage, such as plane shift or forcecage. While the Save DC column of the DMG table partially addresses this issue (the save DC doesn't change based on spell level), it will be up to your judgment as to whether such spells are too strong for a CR3 hag.

Ultimately, the true answer is the least satisfying: you'll have to test it out yourself (DMG 275):

After seeing your monster in action, you might want to adjust the challenge rating up or down based on your experiences.

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  • \$\begingroup\$ This answer implies that a hag that can outright remove a PC from the battle each turn via spells like forcecage, plane shift, or maze, or even potentially the entire party at once via mass suggestion, is no more difficult than one that lacks such spells. That seems like a dangerous implication. \$\endgroup\$ – 8bittree Aug 2 '17 at 20:59
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    \$\begingroup\$ @8bittree, that is true, but that is a fundamental limitation of how the DMG handles the impact of such spells on CR--namely, it doesn't. Anyway, CR should only be a rough guide of how fights will actually go. \$\endgroup\$ – Icyfire Aug 2 '17 at 21:47
  • \$\begingroup\$ But for a question with no mention of RAW, but that is tagged balance, it seems a great disservice to completely ignore everything that doesn't deal or absorb damage, but can easily disable PCs. The How to Answer page even says, "do try to mention any limitations, assumptions or simplifications in your answer." This answer would be improved if it explicitly stated that some non-damage-dealing or -absorbing spells can have a substantial effect on encounter difficulty even though they don't affect CR. \$\endgroup\$ – 8bittree Aug 3 '17 at 16:02
  • \$\begingroup\$ @8bittree That's fair enough--I'll edit that discussion into my answer. \$\endgroup\$ – Icyfire Aug 3 '17 at 16:33
  • \$\begingroup\$ Much better: +1. \$\endgroup\$ – 8bittree Aug 3 '17 at 16:51
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Unfortunately, there is not a clean 1 to 1 ratio between Spell Levels and CR. You can see this quite clearly by perusing the NPCs section of Volo's Guide to Monsters, focusing on pure-casters.

The Abjurer is CR 9 and casts 7th level spells

The Apprentice Wizard is CR 1/4 and casts 1st level spells

The Bard is CR 2 and casts 2nd level spells

Conjurer: CR 6, 5th level spells.

Diviner: CR 8, 8th level spells.

Enchanter and Transmuter: CR 5, 5th level spells.

Evoker and Necromancer: CR 9, 6th level spells.

Illusionist: CR 3, 4th level spells

As you can see, you can't clearly and obviously map CR to Spell Level because different spells of the same level can have dramatically different impacts on CR, based on which spell it is. This only makes sense...a Wizard with Meteor Swarm prepared is going to be a lot more directly lethal in combat than a Wizard with Astral Projection prepared.

So, just to give a more thorough examination, let's have a look at the Evoker. He's a CR 9. determining his defensive CR and half his offensive CR is easy.

Defensive CR is derived from an AC of 15 (CR 5), and 66 HP (CR 1/2). Due to the gap, we adjust the hp based CR two steps up (5 steps different, +1 to HP CR for every 2 steps different between HP and AC CR). So his defensive CR is 2.

In order for him to be CR 9, then his Offensive CR must be 16 ([2+16]/2 = 9). Let's do a bit of math to see if we're right. The Save components of his offensive CR tops out at CR 7. Chain Lightning (his 'punchiest' spell) fired into a crowd averages 200 damage in a single round (assuming it hits all 4 targets, which is the assumption made), in terms of damage dealt...that's a damage component of CR 24.

Damage Component CR of 24 is 17 steps higher than the Save component, so we subtract 8, giving us an Offensive CR of: 16. Exactly what we expected to see.

Thus, taking the average of 16 and 2, we get a CR of 9. Which is exactly what the Evoker is statted at.

So, you can see that when considering a spellcaster's CR, we compute their CR based on the damage output and Save of their punchiest spell.

Thus...if you want to add a damage dealing spell and not alter a creature's CR, then the Damage and Save of that spell must work out to be equivalent to the monster's already existing CR, so you aren't changing their Offensive CR.

So, for a CR 3 monster, the simplest effect would be to give them a spell that deals 21-26 damage on average, with a DC of 13. (You can play with these values in accordance with the Offensive CR guidelines in the DMG)

Unfortunately, even this guideline breaks down when you start looking at Crowd Control spells, or buff/debuff spells. It's hard to quantify how much of an impact these spells can have on combat because, intelligently used, crowd-control magic is devastating even if it doesn't do any damage.

It's at this point that, honestly, experience as a DM comes into play. Spells that make PCs switch teams can potentially boost the enemy's effective CR by an amount equivalent to whatever the CR of that PC would be. Spells that crowd control potentially increase the enemy's CR because there's the potential that it will lock PCs out of the fight.

So, for here...looking at Volo's Guide for a rule of thumb, I would say this as a rough and general guideline. This is NOT a perfect solution, but it's a stopgap that should work until you can build up enough DMing experience to handle it by gut-feel. If you are giving a monster a non-damaging spell that is useful in combat, do not give them a spell that is of a level higher than their CR.

So...

TL;DR

If you want to give them a damage-dealing spell, consider the impact of that spell on the Offensive CR table on DMG page 221. If you want to give them a combat-applicable but non-damaging spell, this will mostly be up to your judgement...but I would strongly recommend against giving them a spell of higher level than their CR. If you want to give them a purely utility spell that has no real impact on combat...do whatever.

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CR is equivalent to fighting 4 players of the same level, e.g. CR3 hag vs 4 3rd level players. So an appropriate CR spell effect would be one equivalent to or just above the spell level players would have access to. A level 3 wizard would have access to 1st and 2nd level spells. So the hag having a 2nd level spell is appropriate. If you wanted to make things a bit more difficult, then maybe a 3rd level spell for the effect.

In short, make it equal to or one level higher than the highest spell level a typical magic user would have for that CR.

When I say equivalent, I mean a 1v4 fight with the 1 being a single enemy. As for the spell effects, this is merely a suggestion for determining what would seem reasonable. A CR5 enemy using 9th level spells is not rational, but using 4th level spells is more reasonable.

In the end, it is up to the GM to determine what would be reasonable.

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    \$\begingroup\$ -1, this is not true. For example, the Archmage in the MM is CR 12 but has 9th level spells. CR is a lot more complicated than being equivalent to the players. \$\endgroup\$ – Icyfire Aug 2 '17 at 15:02
  • \$\begingroup\$ @guildsbounty What he said is right. A single monster of CR=ACL is a CR+0 fight (what you're calling an Normal Fight, and what the DMG calls an average fight.) What you're describing (4 monsters of CR=ACL) is a CR+4 fight. CR+4 fights have roughly a 50% chance of TPK (depending on optimization.) But he's not describing a 4 on 4. He's describing a 1 on 4. \$\endgroup\$ – Byrel Mitchell Aug 2 '17 at 15:14
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    \$\begingroup\$ @guildsbounty Eh, maybe I'm interpreting him wrong. His answer is poorly worded. I was taking "CR is equivalent to fighting 4 players of the same level" to mean "A normal encounter has a single CR monster fighting four players of the same level". If that's what he means, he should edit is answer to be clearer... \$\endgroup\$ – Byrel Mitchell Aug 2 '17 at 15:36
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    \$\begingroup\$ I was referring to 1 monster vs 4 players. When I say equivalent I mean a normal fight for 4 players of level matching the CR. I'm not referring to 4v4. I use the term equivalent to refer to where to gauge CR for a single enemy for the purpose of trying to determine CR appropriate spell effects. I apologise for the confusion \$\endgroup\$ – saxdude1 Aug 2 '17 at 15:38
  • \$\begingroup\$ @saxdude1 that clears that up, upvoted and thanks. However, ideally, your answer should not contain "Edit:" callouts. That's what revisions are for. This is explicitly different from how forums behave in the same circumstance because this site does not operate like a forum. \$\endgroup\$ – godskook Aug 2 '17 at 15:49

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