Does the save DC of a spell being concentrated on change if the caster's Int stat changes in the meantime due to polymorphing?
For instance, if a level 8 wizard with Int 16 casts Hold Person on someone, the save to stop being paralysed is 14.
If the target fails its first save, and then on the next turn, the wizard is polymorphed by another spellcaster into a T-Rex (with Int 2) that is still concentrating on the spell, is the save DC altered for the target's next save against Hold Person?
Best Answer
OPINION: No. The spell DC would not change.
The spell DC is set at the time of the casting, it is not a dynamic feature of the spell. I would think of it like a poison - whose save DC would not change if the venomous creature were polymorphed after delivering the poison. The spell retains the potency it had at its application.
I can find no direct justification of this beyond the fact that changing DCs does not add to game balance and only complicates gameplay. 5e tends to be more streamlined, and tries to avoid dynamic changes mid-combat.
I don't think you can get a non-opinion based answer unless you ask Crawford of Mearls. I searched the PHB, Sage Advice, and the DMG. The various spell entries, combat rules and spell rules have nothing to say on this that I could find, unfortunately.
That said, it would not be the end of the world to do it the other way. Just a little more book keeping at the table.