# How would permanent invisibility affect a monster's challenge rating?

I’m attempting to convert a phantom fungus to fifth edition. The creature is always invisible while alive, similar to an invisible stalker. When calculating challenge rating, how would this invisibility affect the value?

In the “Monster Features” table (Dungeon Master's Guide p. 280), the Invisibility listed is from the imp's stat block which differs from the invisible stalker's feature. My guess would be that offensively it would be similar to Pack Tactics as attacks while invisible are made with advantage and defensively it would be similar to Superior Invisibility, though without concentration. So, would the following make sense?

• Increase the monster’s effective attack bonus by 1.
• Increase the monster’s effective AC by 2.
• Challenge rating calculations are different between 3.X and 5ed. These are practically separate games. Please decide on one edition. Commented Nov 25, 2021 at 22:25
• I am attempting to determine the 5e CR for this creature, but more generally how this type of permanent invisibility would effect any fifth edition creature's CR. Commented Nov 25, 2021 at 22:30
• I've removed he 3.5 tag to remove confusion. Commented Nov 25, 2021 at 22:48
• Are you looking for what the Monster Manual calucation would use, or what we believe the designer's would rate it? I ask because they don't use the formula they lay out, and so the answer might be different depending on what you're looking for. Commented Nov 25, 2021 at 22:55
• I was looking for how the calculation would work according to the "Creating a Monster" section of the DMG in the general case, but I understand that playtesting is important in determining the actual CR. Commented Nov 25, 2021 at 23:04

### Permanent invisibility increases Challenge Rating by 3.

#### Assuming (dis)advantage is ±5 to a roll, CR increases by 2 before accounting for the invisibility being permanent.

The conventional wisdom is that advantage and disadvantage roughly translate to a ±5 to a d20 roll (for more details see the last section of this answer). Taking this rough assumption, we can apply it to the Challenge Rating guidance found in the Dungeon Master's Guide:

Defensive Challenge Rating. [...] If your monster’s AC is at least two points higher or lower than that number, adjust the challenge rating suggested by its hit points up or down by 1 for every 2 points of difference.

Offensive Challenge Rating. [...] If your monster’s attack bonus is at least two points higher or lower than that number, adjust the challenge rating suggested by its damage output up or down by 1 for every 2 points of difference.

Assuming then that having disadvantage in attacks against the creature correlates to +5 Armor Class, and the creature having advantage on all attacks correlates to +5 to the bonus to hit, the result is a +2 to both Defensive and Offensive Challenge Rating, and since overall Challenge Rating is the average of these, we net a +2 to overall Challenge Rating. However, this is before we account for the permanency of the invisibility.

#### +2 CR is consistent with guidance given for similar features, but I suggest +3 because permanent invisibility is superior to these features.

Unfortunately, the Challenge Rating system doesn't encode every property of a creature. Fortunately, we have some guidance given for similar features in the Monster Features Table. The Goblin has the Nimble Escape feature, and the guidance given in the table matches the intuition I gave above:

### Monster Features

Name Example Monster Effect on Challenge Rating
Nimble Escape Goblin Increase the monster’s effective AC and effective attack bonus by 4 (assuming the monster hides every round).

This feature allows a goblin to Hide as a bonus action each around, which can result in similar circumstances as permanent invisibility - advantage on attacks by and disadvantage on attacks against the creature:

Nimble Escape. The goblin can take the Disengage or Hide action as a bonus action on each of its turns.

This nets a +2 to Challenge Rating for the reasons described in the first section.

However, taking the Hide action requires rolling a Dexterity (Stealth) check, and so is not guaranteed to work every time. Permanent invisibility is guaranteed to work, so it is clearly superior to Nimble Escape. Additionally, permanent invisibility also provides the other potential benefit of Nimble Escape as it prevents opportunity attacks; the rules for opportunity attacks state:

You can make an opportunity attack when a hostile creature that you can see moves out of your reach.

Therefore, I suggest +3 to CR, instead of +2.

• "This feature allows a goblin to Hide as a bonus action each around, which can result in similar circumstances as permanent invisibility - advantage on attacks by and disadvantage on attacks against the creature" "Permanent invisibility is guaranteed to work, so it is clearly superior to Nimble Escape." A successful Hide does more than give disadvantage on attacks; since the creature can move after hiding, it can often mean complete untargetability (granted this requires 2+ contiguous heavily obscured squares). As such, it's misleading to say Invisibility is strictly superior. Commented Nov 28, 2021 at 9:01
• @Vigil I’m failing to see how permanent invisibility doesn’t also work that way most of the time. Commented Nov 28, 2021 at 14:22
• To use permanent invisibility to conceal your position (especially after e.g. an attack gives it away), you need to take the Hide action (see rpg.stackexchange.com/questions/86945/…). The Goblin has the action economy (bonus action Hide) to do that every turn after attacking. A creature with only this invisibility feature does not, as to conceal its position it must use its main action to Hide. Commented Nov 29, 2021 at 15:18

Superior Invisibility is listed on the chart, referencing the Faerie Dragon. It isn't quite the Stalker's ability, but turning invisible as a bonus action (with concentration) means the monster will virtually always be attacking from invisibility, and at minimum will be invisible to the first attack against it each turn.

The chart only specifies an effective +2 to AC, which seems strange to me, but that's how it's listed. Possibly they're taking into account that there are a number of ways to bypass invisibility.

• I chose not to mention it in my answer because I’m pretty sure the table is just plain wrong on that one (for all the reasons explained in my answer). Commented Nov 28, 2021 at 1:26