How does a ghost's Horrifying Visage aging interact with immunity to the frightened condition?

Question

Assume Bob the Brave is immune to the frightened condition.

He is faced with a ghost, that attempts to use Horrifying Visage against him.

Does his immunity prevent him from rolling the save, failing by 5 or more, and gaining white hair?

Answer 1

It's ambiguous, but I would rule that...

Bob ages.

Immunity to some damage type does not usually prevent other effects of the spell, as per this, and I think it is logical to do the same for conditions. This is what Horrifying Visage says about its frighten effect:

A frightened target can repeat the saving throw at the end of each of its turns, ending the frightened condition on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to this ghost's Horrifying Visage for the next 24 hours.

The aging effect is separate from the fear effect.

If the save fails by 5 or more, the target also ages 1d4 × 10 years. [...] The aging effect can be reversed with a greater restoration spell, but only within 24 hours of it occurring.

Now, here's the ambiguity. You can see that the aging effect depends on failing the save by 5 or more, not on being frightened. But the text uses the word also when referring to the age effect. My interpretation is that the rules simply don't account for edge cases where you could be immune to one of the effects. They would assume that you would fail the save by 5 or more, become frightened, and in addition to that (but not instead of that), age a few years.

Contrast this with effects like the Mummy's Dreadful Glare:

If the target fails the saving throw by 5 or more, it is also paralyzed for the same duration.

The wording is different, and the paralysis lasts for the duration of the frightened condition. If you can't be frightened, then you can't be paralyzed by that feature.

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Bob rolls the save. Regardless if he fails or not, he is not frightened (he is immune, after all). Other creatures that had failed the save, would lose the fear effect by eventually rolling a successful save in the following turns and becoming immune to the feature. However, they would still have aged.

If Bob failed the save by 5 or more, it would be the same as a creature that failed the save by 5 or more, and then succeeded on the save in the following turn. That creature would be immune to Horrifying Visage, and no longer frightened, but would still have aged. Bob has 24h to use a Greater Restoration on himself. Regardless of his bravery.

Answer 2

Bob doesn't need to roll the save, due to the immunity to being frightened.

Horrifying Visage states:

Each non-undead creature within 60 feet of the ghost that can see it must succeed on a DC 13 Wisdom saving throw or be frightened for 1 minute. If the save fails by 5 or more, the target also ages 1d4 × 10 years. A frightened target can repeat the saving throw at the end of each of its turns, ending the frightened condition on itself on a success. If a target's saving throw is successful or the effect ends for it, the target is immune to this ghost's Horrifying Visage for the next 24 hours.

The saving throw is primarily to avoid being frightened, which as Bob is immune, he can be considered to have automatically succeeded (due to a failure resulting in the frightened condition).

Further, the text says if the save fails by 5 or more, the target also ages. 'Also' means 'in addition to'. So the frightened condition must occur first, in order for the aging to apply.

Finally, because the saving throw is primarily to avoid being frightened, and being immune to that condition is equivalent to an automatic success, Bob would be immune to the Horrifying Visage for the next 24 hours, further cancelling any risk of effect aging.

To better understand the interaction, try reading the results of failure without the word 'also':

Each non-undead creature within 60 feet of the ghost that can see it must succeed on a DC 13 Wisdom saving throw or be frightened for 1 minute. If the save fails by 5 or more, the target ages 1d4 × 10 years.

Notice the subtle but distinct difference that removing the word 'also' makes, as the effect of failing by 5 or more now reads to be completely independent of becoming frightened, giving the saving throw 2 distinct possible results of failure.

Another way to consider it, is to remove the first part, and focus only on the second (especially since the frightened condition cannot occur in this case):

If the save fails by 5 or more, the target also ages 1d4 × 10 years.

Seen this way, it raises the question, what is 'also' referring to? Basically, the second part is an incomplete statement, dependent upon the first part (the frightened condition). Without the first part, the second is nonsensical, and cannot apply.

Answer 3

Bob the Brave does not age

We can come to this conclusion directly if we examine what a Wisdom saving throw is meant to represent. The Dungeon Master's Guide Saving Throws subsection, of the Using Ability Scores section, provides us with some key information.

A Wisdom saving throw is used for:

Resisting effects that charm, frighten, or otherwise assault your willpower.

The Ghost's Horrifying Visage action is worded thus:

Each non-undead creature within 60 feet of the ghost that can see it must succeed on a DC 13 Wisdom saving throw or be frightened for 1 minute. If the save fails by 5 or more, the target also ages 1d4 x 10 years. A frightened target can repeat the saving throw at the end of each of its turns, ending the frightened condition on itself on a success. If the target's saving throw is successful or the effect ends for it, the target is immune to this ghost's Horrifying Visage for the next 24 hours. The aging effect can be reversed with a greater restoration spell, but only within 24 hours of it occurring.

The Wisdom saving throw from the Ghost's ability is clearly being used to resist being frightened.

We can do some further analysis by transforming how this ability is worded into a more general from:

1. Targeting criteria

2. Target makes a Wisdom saving throw, if it can see the ghost

3. If that saving throw fails by 5 or more a thing happens in addition to being frightened

4. Some text on what happens on a successful saving throw re immunity to the feature

5. If required, how you reverse the effects.

Specifically looking at the generalised version of the "ancillary" conditional wording structure:

If the [saving throw] fails by 5 or more, [the target is] also [affected by some additional effect]

This wording is pretty clearly implementing the Degrees of Failure consequence from Dungeon Master's Guide Resolution and Consequences subsection of Using Ability Scores.

Degrees of Failure

Sometimes a failed ability check has different consequences depending on the degree of failure. For example, a character who fails to disarm a trapped chest might accidentally spring the trap if the check fails by 5 or more, whereas a lesser failure means that the trap wasn't triggered during the botched disarm attempt. [...]

Which supports the interpretation that the creature is so frightened (ie they failed the saving throw by a significant amount) that they are afflicted with some additional effect.

If you are immune to being frightened you can't be more frightened, or so frightened, that you suffer an additional effect. Thus Bob the Brave does not age.

Answer 4

It is unclear from an isolated reading of the Ghost stat block and lore. There is one other monster ability that we can use to conclude that Bob the Brave does not age

We first need to do a bit of textual analysis on the Ghost's ability, and a similarly worded ability of another monster (the Mummy's Dreadful Glare) to pick out similarities which can help us with our interpretation of the Ghost's ability.

The Ghost's Horrifying Visage action is worded thus:

Each non-undead creature within 60 feet of the ghost that can see it must succeed on a DC 13 Wisdom saving throw or be frightened for 1 minute. If the save fails by 5 or more, the target also ages 1d4 x 10 years. A frightened target can repeat the saving throw at the end of each of its turns, ending the frightened condition on itself on a success. If the target's saving throw is successful or the effect ends for it, the target is immune to this ghost's Horrifying Visage for the next 24 hours. The aging effect can be reversed with a greater restoration spell, but only within 24 hours of it occurring.

The Mummy's Dreadful Glare feature is worded:

the mummy targets one creature it can see within 60 feet of it. If the target can see the mummy, it must succeed on a DC 11 Wisdom saving throw against this magic or become frightened until the end of the mummy's next turn. If the target fails the saving throw by 5 or more, it is also paralysed for the same duration. A target that succeeds on the saving throw is immune to the Dradful Glare of all mummies (but not mummy lords) for the next 24 hours.

Both abilities are worded in this manner:

1. Targeting criteria

2. Target makes a Wisdom saving throw, if it can see the ghost/mummy

3. If that saving throw fails by 5 or more a thing happens in addition to being frightened

4. Some text on what happens on a successful saving throw re immunity to the feature

5. If required, how you reverse the effects.

In particular both abilities have the exact same "ancillary" conditional wording structure:

If the [saving throw] fails by 5 or more, [the target is] also [affected by some additional effect]

As an aside this wording is pretty clearly implementing the Degrees of Failure consequence from Dungeon Master's Guide > Using Ability Scores > Resolution and Consequences.

Functionally this wording supports an interpretation that the creature is so frightened (ie they failed the saving throw by a significant amount) that they are afflicted with some additional effect.

That being said, from a purely textual analysis, this wording is not particularly clear on what happens if a creature is immune to the first condition. Do they still need to make the saving throw? Can they fail the saving throw, and by extension fail it badly, given they are immune to being frightened?

Thankfully, however, there is some additional lore given for the Mummy, which can help us parse this ability wording consistently:

Creature of Ritual. [...] The overwhelming terror that foreshadows a mummy’s attack can leave the intended victim paralyzed with fright. [...]

This lore segment makes it clear from a mechanics perspective that the paralysis caused by the Mummy's dreadful glare ability in the stat block is caused by the targeted creature being so frightened they are paralysed. Since Bob the Brave cannot be frightened, he cannot be frightened into a state of paralysis.

By the principle of there being no hidden rules, monster abilities that are worded in the same way should function in the same way.

As a result, we can use the clarity we have on the Mummy ability, to help parse the Ghost ability. In particular the Ghost ability should function the same way as the Mummy ability when applying the ancillary condition on an additional phase.

As a result, Bob the Brave cannot be frightened so much that they spontaneously age because Bob the Brave is immune to being frightened.

Answer 5

Bob doesn't age

According to the Ghost stat block:

Horrifying Visage. Each non-undead creature within 60 feet of the ghost that can see it must succeed on a DC 13 Wisdom saving throw or be frightened for 1 minute. If the save fails by 5 or more, the target also ages 1d4 × 10 years.

For it to age Bob, he would have to fail the frightened check. However, according to the PHB:

A saving throw — also called a save — represents an attempt to resist a spell, a trap, a poison, a disease, or a similar threat.

There is no attempt, he is immune by default.
And continues:

You don’t normally decide to make a saving throw; you are forced to make one because your character or monster is at risk of harm.

But with Bob, he isn't at risk of harm. There is no way for him to become frightened, so there isn't any chance of it working.

The DMG also says:

A save makes the most sense when something bad happens to a character and the character has a chance to avoid that effect.

In this case, there is nothing bad happening to him, and there is no chance to avoid an effect as he is automatically immune the outcome.

Answer 6

I think the best way to answer this is by comparing it with a similar feature that utilises the target, or a creature in general, being frightened. For this, I'll be looking at the Mummy and their Dreadful Glare feature:

The mummy targets one creature it can see within 60 feet of it. If the target can see the mummy, it must succeed on a DC 11 Wisdom saving throw against this magic or become frightened until the end of the mummy's next turn. If the target fails the saving throw by 5 or more, it is also paralyzed for the same duration. A target that succeeds on the saving throw is immune to the Dreadful Glare of all mummies (but not mummy lords) for the next 24 hours.

In a linked question that look at this feature, there were a few things discussed about the relationship between being frightened and paralysed, I will confidently side with the fact that if you are immune to being frightened, you are also immune to being paralysed, not because of any flavour-based reason like the accepted answer on that question suggests, but instead because of the language used:

"it is also paralyzed for the same duration..."

The duration of you being frightened would be, not at all, therefore the duration of you being paralysed would be, not at all.
If we approach this question with the same assumption however, there is no mention of duration when it states:

"the target also ages 1d4 x 10 years..."

This leads me to believe that Bob would get all the wrinkles.

To explain more, Bob has immunity to the frightened condition, but that doesn't mean he has immunity to failing the saving throw by very large amounts. You asked "Does his immunity prevent him from rolling the save, failing by 5 or more, and gaining white hair?", no because Bob doesn't have immunity to gaining white hair, unless you're using homebrewed rules you haven't mentioned.
Bob rolls a 5 against the 13DC Wisdom save, he isn't frightened, however we have to continue through the feature as you would with a character who could be frightened as there is nothing mechanical to say "Stop here if you aren't frightened". Bob has failed by more than 5, therefore he ages (1d4 x 10) years. He's not frightened so he doesn't have to repeat the save, but he is now immune to Horrifying Visage for the next 24 hours. If at this point we were to hypothetically force Bob into making another saving throw with this new immunity effect, he would finally have the aformentioned "immunity to gaining white hair", this would be the only way to both fail the save by more than 5 and avoid ageing.

To address a comment on the accepted answer: "DMG p. 237 has some points on "don't roll if the outcome is not in doubt", the outcome consists of more than one component, there is the condition contracted by failing the save, and if that was the only thing I'd agree that you didn't even have to roll, however because there is another outcome, what happens when you roll 5 or more under the DC, I'd say that's a variable outcome, prompting the role despite this rule in the Dungeon Master's Guide.

But you're the DM:

I assume anyway... If you are, you can decide that based on the flavour of the feature and how the creature is written and how you would describe the feature, the character is wholly immune to the feature, as the original poster of the linked question decided when they marked a similar answer as correct. My answer is based on loyally following the rules as they are written and any assumptions I make are from the perspective of a British English speaker making very very basic language analysis. Hope this provides sufficient evidence for my answer.