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?
Best Answer
Phantasmal Killer has been errata'd http://media.wizards.com/2015/downloads/dnd/PH_Errata_1.1.pdf
So the complete exact text is:
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
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.