Inspired by this contentious [question][1].

The DMG indicates that to calculate a creature's save DC, the following formula should be used:

> 8 + Monster's Proficiency Bonus + Relevant Ability Score Modifier

Based on the criteria to calculate a creature's Save DCs, dragons' Frightful Presence save DC is based upon their Charisma modifier, which seems appropriate considering the manner in which the monster feature is supposed to work. Failing the save against this leaves the character Frightened for 1 minute or until they successfully save.

Suppose Bob the Fighter fails his save against an Ancient Blue Dragon's Frightful Presence (CR 23, Cha Mod +5, therefore save DC 20) and elects to continue fighting the dragon because he knows that with a +2 on his Wisdom save, he's never going to succeed and 1 minute is too long to be out of the fight. Subsequently, Melv the Wizard manages to connect with a Polymorph spell on the dragon and turns the dragon into a Frog (CR 0, Cha Mod -4, save DC ??).

Several questions from this:

 1. Does Bob need to continue making saves against Frightful Presence? Polymorph does not say it removes the monster's features and it is not like the Change Shape feature, which specifically specifies what features the dragon loses when using that ability.
 2. If Bob does need to continue making saves. What is the DC? Does it remain at 20 because that was the initial DC? Does it become 11 because the dragon's charisma modifier has been dropped by effectively 9 points, but the dragon retains the proficiency bonus? Or does the dragon also lose the proficiency bonus and the DC becomes 6?

What happens to Bob?

  [1]: https://rpg.stackexchange.com/questions/105855/does-the-save-dc-of-a-spell-decrease-if-the-original-spellcaster-polymorphs-on-a/105860#105860