Prismatic Spray is an area of effect spell that affects each creature in a 60 foot cone with a random colored ray. The Red, Orange, Yellow, Green, and Blue rays are all resolved on the turn the spell is cast, while the Violet ray is fully resolved at the start of the caster's next turn.
The indigo ray says (emphasis mine):
On a failed save, the target is restrained. It must then make a Constitution saving throw at the end of each of its turns. If it successfully saves three times, the spell ends. If it fails its save three times, it permanently turns to stone and is subjected to the petrified condition. The successes and failures don't need to be consecutive; keep track of both until the target collects three of a kind.
A failed save against the indigo ray thus takes between three and five of the target's turns to fully resolve. Because multiple targets may be within the area of effect, there may be multiple targets simultaneously under the effects of different indigo rays from the same spell. More rarely, a single target may be under the effects of multiple indigo rays from the same spell (1/392nd chance per target that fails the initial Dex save).
If an indigo ray target makes three successful Con saves, the spell ends. Apparently the 'entire' spell ends, not the specific effect of a single ray on a single target.
Does this mean that for two targets currently making saves against indigo rays, the third success by one target would end the spell effect on the second target, obviating both the Restrained condition and the need to continue to make saves? Note that this is asymmetric in the sense that three failed saves will petrify only the creature failing the saves, but three successful saves could save all creatures still making saves. If true, that means that the more targets are hit by indigo rays, the more likely each target is to escape petrification, as there is an increasing chance it will be saved by another target ending the spell.
Further, if a target struck by an indigo ray was also struck by a violet ray and transported to another plane, would the 'connection' between indigo rays persist, so that whomever made three saves first would end the spell on the other indigo-targets, even across planes?
I understand that there is a difference between the spell and the spell effect, but this does not seem to be like the case of dispel magic against a spell cast on multiple targets, because whereas dispel magic says (emphasis mine):
Any spell of 3rd level or lower on the target ends
prismatic spray says a third successful save ends the spell, not the spell on a target.
I understand that some spells have effects that persist that are not part of the spell itself, like the lethargy that follows the end of a haste spell. In this case, it appears to me that the potential of petrification, since it is ended with the end of the spell, is not an effect that persists beyond the spell.
Do three saves by any target actually end the spell for all targets?
Best Answer
This looks like another case of "reason vs strict text reading"
As written, you are correct : the spell ends, meaning all its effects are terminated, including ongoing effects of any ray color on any other creature.
However, it stands to reason that this makes no sense within the context of the spell : a zone spell shouldn't stop just because one person in its area is not affected by it anymore or resisted its effects. This looks to me like an oversight in the way the text was written.
In the end, it's up to the DM to decide on how to rule this. I see little to no reason to decide that the whole spell would end if one person resists its effects, though.