Mind Sliver (Tasha's Cauldron of Everything p.108) says:
The target must succeed on an Intelligence saving throw or take 1d6 psychic damage and subtract 1d4 from the next saving throw it makes before the end of your next turn.
I am confused about the timing of the "next saving throw". If I used Mind Sliver on a Concentrating creature and it failed its save, the d6 psychic damage would provoke a Constitution save to maintain Concentration.
Would this save count as its "next saving throw" and be immediately affected by the penalty from the Sliver? That is, the damage and the penalty application get resolved at the same time, and then the damage triggers the saving throw, whose resolution is affected by the pre-existing penalty.
Or would the penalty be applied only after all the effects of the Sliver, including the damage and an unpenalized forced Concentration save, had been resolved? That is, the damage and the penalty application are initiated simultaneously, but then the damage triggers the saving throw, which must be resolved before the penalty finishes resolving and thus is made unpenalized.
If the simultaneous effects rule from Xanathar's is in play, is the creature whose turn it is (most likely the caster of the mind sliver) permitted to resolve the damage and the throw penalty as two separate events, in the order of their choosing, and would that then affect the answer to this question?