Does Mind Sliver's saving throw penalty apply to Concentration saves from its own damage?

Question

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?

Answer 1

A strict reading leads to a penalized Concentration check.

As you reported, the spell description says (emphasis mine):

[...] and subtract 1d4 from the next saving throw it makes before the end of your next turn.

A strict reading of the meaning of next leads to applying the penalty to the very first\$^1\$ saving throw done before the end of your next turn. Since the damage and the penalty take place at the very same time when Mind Sliver affects the target, the first saving throw done before the end of the caster's next turn is the Concentration check: such a check shall thus suffer from the penalty.

A loose reading of spell's description may leave room for deciding the order of effects.

Nonetheless, a more elastic interpretation of the description of the spell might lead to the concerns about the order of the effect that you explained above. This can be resolved in two ways:

• The DM decides.
• XGtE's rules are applied and the decision is made by the creature whose turn it currently is. Note that usually such a creature is the caster, but it could be also another creature's turn (even the target's) if Mind Sliver is cast as a reaction.

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\$^1\$ See for example the following definition:

• immediately adjacent (as in place, rank, or time) [reference]