The description of the synaptic static spell (XGtE, p. 167) reads:
After a failed save, a target has muddled thoughts for 1 minute. During that time, it rolls a d6 and subtracts the number rolled from all its attack rolls and ability checks, as well as its Constitution saving throws to maintain concentration.
Does the target make a d6 roll for each attack/check/Con save roll, or does it make one d6 roll at casting time?
One of my players (the wizard who cast the spell) argues that the d6 is rolled once when the spell is cast, and the result is applied for the duration, while I suspect the d6 was meant to be rolled each time separately.
The reason for the debate comes from the uncommon wording, when compared to spells such as bless:
Whenever a target makes an attack roll or a saving throw before the spell ends, the target can roll a d4 and add the number rolled to the attack roll or saving throw.
Best Answer
The wording is ambiguous, so we favor the established precedent of separate rolls
As you say, the grammatical structure of the spell's description is ambiguous. It only mentions a single d6 and a single number rolled, but it's entirely possible to read it as a single d6 roll per d20 roll. Consider that in common RPG lingo, "subtract a d6 from all rolls" would conventionally mean doing a separate d6 roll for each roll that you're subtracting from.
However, there is an established pattern of dice usage in D&D that we can use to resolve the ambiguity: dice rolls are used once and then forgotten. Whenever a die is rolled, it is used to resolve some instantaneous effect, and then the roll can safely be forgotten. For example, if I roll a 19 on my wisdom saving throw against Charm Person and the DC is 16, all I need to remember is the result: I succeeded on the saving throw and am not charmed. I don't need to remember that I rolled a 19, and I don't need to remember that the DC was 16. Similarly, every time I take damage, I just need to update my current HP (and possibly make a concentration check based on the damage amount), and then I can forget what the damage roll was.
One exception to this pattern is the Divination wizard's Portent feature:
This feature allows the player to roll two dice and save them for later. But even these rolls are each used only once and do not need to be remembered once used. The only difference is that the rolling and use are separated in time.
The point is, the rules always assume, unless they tell you otherwise, that all dice rolls are used once and then forgotten immediately after resolving the effect that they mediate. And in the rare case where the rolling and the use of that roll are separated in time, the rules will say to record the roll for later use. The text of Synaptic Static does not say to record the d6 roll, and it doesn't unambiguously say that the roll should be reused.
So grammatically, the spell's text is ambiguous, with two equally valid-seeming interpretations: roll one d6 and reuse the roll for the duration, or make a separate d6 roll for each affected d20 roll. One of these interpretations would introduce a new unprecedented mechanic (reuse of the same die roll at different times) while the other one sticks to established mechanics (each die roll is used once and discarded). The reasonable ruling is to prefer the established precedent, rather than introducing a new mechanic via grammatical ambiguity. With this ruling, a separate d6 roll is made and subtracted for every attack roll, ability check, and concentration check made by an affected creature.