Does the Phantasmal Killer spell affect creatures immune to being frightened?

Question

The description of the phantasmal killer spell says (PHB p. 265, emphases mine):

You tap into the nightmares of a creature you can see within range and create an illusory manifestation of its deepest fears, visible only to that creature. The target must make a Wisdom saving throw. On a failed save, the target becomes frightened for the duration. At the start of each of the target’s turns before the spell ends, the target must succeed on a Wisdom saving throw or take 4d10 psychic damage. On a successful save, the spell ends.

My question is regarding the damage that the creature takes each round. The spell description doesn't cover what happens if the creature becomes immune to being frightened, or what happens if someone removes the frightened condition from the target, etc.

Per RAW and per RAI, does phantasmal killer's psychic damage affect creatures immune to being frightened? And if the creature becomes immune during the spell duration? And what happens if someone removes only the frightened condition?

Answer 1

Phantasmal Killer has been errata'd http://media.wizards.com/2015/downloads/dnd/PH_Errata_1.1.pdf

Phantasmal Killer (p. 265). The frightened target makes a save at the end of its turns, not the start

So the complete exact text is:

You tap into the nightmares of a creature you can see within range and create an illusory manifestation of its deepest fears, visible only to that creature. The target must make a Wisdom saving throw. On a failed save, the target becomes frightened for the duration. At the start end of each of the target’s turns before the spell ends, the target must succeed on a Wisdom saving throw or take 4d10 psychic damage. On a successful save, the spell ends.

IMHO DM Ruling is required here.
That successful save refers to any save for that spell or just one of the "avoid the damage" saves?

Since the errata says "the frightened target makes a save" I'd also rule that if the target has the frightened condition removed the entire spell ends.

Please note that this strict ruling makes this 4th level spell sub-part.
Feel free to give Phantasmal Killer some DM love :D

Edit:
After more general considerations and a lenghty discussion with friends I feel the need add this:

Lets consider Vicious Mockery

You unleash a string of insults laced with subtle enchantments at a creature you can see within range. If the target can hear you (though it need not understand you), it must succeed on a Wisdom saving throw or take 1d4 psychic damage and have disadvantage on the next attack roll it makes before the end of its next turn.

If cast on a creature immune to psychic damage should I roll the save? Yes, because a failed saving throw determines two outcomes, psychic damage and disadvantage on the next attack roll. If the target fails the save it would take no damage but still suffer the disadvantage.

So, back to Phantasmal Killer:
"On a successful save, the spell ends." seems pretty clear to me. If the save agains fear succeeds the spell ends, if it fails the spell goes on even if the target is immune to the frightened condition. This is brutally RAW.

I very much doubt this is RAI, especially considereing the wording in the errata. In any case, forgoing the first save its a bit too much.

Answer 2

Yes. The frightened condition and the damage caused by the spell are separate.

I read the (amended) spell description as follows:

You tap into the nightmares of a creature you can see within range and create an illusory manifestation of its deepest fears, visible only to that creature.

• The target must make a Wisdom saving throw. On a failed save, the target becomes frightened for the duration.
• At the end of each of the target’s turns before the spell ends, the target must succeed on a Wisdom saving throw or take 4d10 psychic damage. On a successful save, the spell ends.

Crucially, successful save in the last sentence refers only to the Wisdom saving throw mentioned in the previous sentence; it's describing the corresponding outcome for success to the clause describing failure in the preceding sentence.

Thus, nothing in the spell description makes the psychic damage or the spell duration conditional upon the target being susceptible to the frightened condition. Likewise, if the target was immune to psychic damage but not to the frightened condition, they would still be frightened for the spell's duration.

Since the ongoing psychic damage is independent of the frightened condition, that part of the spell continues to apply if the target is frightened and later becomes immune to being frightened, or has that condition removed.

Contrast this with the description of Fear: (PHB, p. 239)

Each creature in a 30-foot cone must succeed on a Wisdom saving throw or ... become frightened for the duration.

While frightened by this spell, a creature must take the Dash action and move away from you by the safest available route on each of its turns, unless there is nowhere to move. If the creature ends its turn in a location where it doesn’t have line of sight to you, the creature can make a Wisdom saving throw. On a successful save, the spell ends for that creature.

Fear is similar to Phantasmal Killer in that it initially imposes a condition on the target(s), and then has additional effects on each of the targets' turns. But, unlike Phantasmal Killer, Fear makes it clear that the ongoing effects are conditional on the initial condition by using the conditional phrase "While frightened by this spell ... ". There is no such condition applied to the ongoing damage effect of Phantasmal Killer, so it doesn't seem reasonable to read the spell as having the first saving throw mitigate all of the further described effects.

Thus, the target of Phantasmal Killer makes a save when the spell is cast, and is frightened if it fails, and they make a save on each of their turns while the spell is in effect. As soon as they succeed on any of the second kind of saves, the spell ends (and it will also end if the caster loses concentration, or one minute passes).

Answer 3

If you have immunity, you just do not get any damage from that type of damage.

Since the spell clearly says you get frighten, it is clear to me that if you are immune to being frighten, then the spell fails (the illusion may still be created and going, but it does not affect you at all).

I understand that the damage says "psychic damage" and not being frighten does not exactly mean you could not get psychic damage, but the illusion is there to create scary nightmares. If you are not scared by such, I really do not see why you would get any damages. You'd just be laughing at those fun dreams...

Plus, this is an illusion spell. Once you see through the illusion (succeed on a save or are immune) the illusion could continue but you would not be affected at all anyway. That being said, the spell clearly says that it ends once you succeed on a saving roll. This means the caster knows once you succeed and can stop concentrating on the spell altogether. This also means that if the target is immune, then you know immediately.

---

As a RAW reference to my answer, the Dream spell (PHB p. 236) also generates psychic damages:

[...] In addition, when the target wakes up, it takes 3d6 psychic damage.

Elves are immune to that spell, as specified in the Thay encounter in The Return of Tiamat p. 77, Dreams and Nightmares section. Elves are not immune to psychic damages nor illusions. What happens is that they do not go to sleep, as they meditate instead, and thus cannot be affected by the side effects of the spell (the 3d6 psychic damage) since the illusion does not take on them in the first place.