Sunbeam's text reads:
A beam of brilliant light flashes out from your hand in a 5-foot-wide, 60-foot-long line. Each creature in the line must make a Constitution saving throw. On a failed save, a creature takes 6d8 radiant damage and is blinded until your next turn. On a successful save, it takes half as much damage and isn't blinded by this spell. […]
You can create a new line of radiance as your action on any turn until the spell ends.
Is my understanding of how this spell works correct?
My interpretation: A creature may be affected by beams over and over. If a creature makes its saving throw, it can no longer be blinded by additional beams. However, on subsequent turns, it still needs to make a Con save against the damage. Is that correct?
Best Answer
Any failed saving throw results in damage and blinding
Each time a creature is targeted with a beam of light from the spell, it must roll the saving throw, and it suffers the effects as normal. Passing any one saving throw does not confer any protection from the effects of failing future saving throws against the same spell. The spell's text specifies that on a successful save, a creature isn't blinded by the spell. This is written in the present tense: it just means that the creature doesn't get blinded right now. There's nothing that says it can't be blinded by future light beams from the same spell.
Generally, if saving against a spell once confers any kind of ongoing protection against that spell, this will be clearly spelled out in the spell's text. For example, the Eyebite spell says (emphasis added):
Sunbeam does not contain any text like this.