[RPG] What are CR-appropriate spell effects


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.

Best Answer

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.