No. Having SB allows you to affect any of those rolls. Therefore we need to compare with subclasses that also has options to affect each of those rolls. I have made no requirement that they have to be able to do it all at once.
Ok, but then you also have to compare number of uses. If you want the ability to affect ANY of these rolls then you can affect at most one of two of these abilities.
For example, if I want to be able to affect saves, attacks and ability checks. I can only affect two attack rolls an entire day. If I affect more than two attack rolls then I can't effect both saves and abilities as well because I only have 4 slots.
The number of rolls you are going to affect matters in the value discussion. Something like Cause Fear is going to cause disadvantage on a lot of attacks and ability checks. Something like Bane is going to cause -1d4 on a lot of ability checks and saves. These are both 1st level spells.
I don't think I've spotted this. How are they doing it?
Any Pact of Chain Warlock with Investment of Chainmaster evocation using his bonus action to have his Sprite attack. No limit on uses.
You can do this with a Quasit as well, and even do it once a day using scare instead of an attack and not even need to use a BA that time. The difference with a Quasit is the Quasit has to go into melee (after the scare use) and the enemy gets to save every turn, where there is no further save against the sprite.
Combine this with the Bane spell (Undead Warlock) and you have someone at 3rd level that on round 1 could use a bonus action to try to give one enemy disadvantage every turn for an entire minute on attacks and abilities and then cast Bane on 4 enemies giving each of them -1d4 on attacks and saves for a minute. That is a ton of rolls being effected using a 1st level slot, an action and a bonus action. He can keep adding more and more enemies every turn.
You have not spotted it because it is not overpowered, despite the fact it can give disadvantage to dozens of rolls a day and affect every single attack, save and ability check an enemy makes turn after turn.
I really need to check your maths here. If you are rolling a D20 twice, with the same DC, and taking any failed result, the chance of failure is identical. It doesn't matter whether you roll them at the same time, or the second D20 only after you see the result of the first.
This is an outright falsehood. The chances of failure are the same. You are rolling a D20 twice at the same DC, and taking any failure as the result. It is also the same chance as if you were to cast the same spell at them on a subsequent round if they make the save the first time.
Because if you are casting silvery barbs he is not rolling a D20 twice, you are only rolling a D20 once.
The other dice is fixed and it is already a success before you cast the spell.
It is more efficient in terms of spell use but it is less powerful than disadvantage because one of the rolls already succeeded before SB is in play. This dramatically affects the chances that taking the reaction casting the spell will cause a failure.
SB is like playing blackjack and going "double or nothing" after your first hand already lost.
For clarity, my statement was: SB will rarely cause an enemy to fail a save he has already succeeded on in game play.
How much experience do you have playing with SB? I admittedly have no experience using or seeing the spell used yet, but I do understand the statistics around it. Unless you have seen it in play being effective for this often then you can't say you have experience that disagrees.
This is one of those
stars have to align situations with a whole string of conditions needed for SB to flip a save - within 60', reaction available, higher level spell cast, SB prepared, slot available,
enemy makes the save AND enemy fails the SB reroll.
That string of things will happen, but very rarely. The overall probability is the product of the individual probability of all those conditions.
The last two things underlined are the big problem with this being used often for this. In the scenario described, they are not independent, they are dependent. If one of these is a high probability the other will be low. The underlined conditions are inversely proportional with a sum of their probability equal to 1 and a product of their probability equal to 0.25 or less.
The dependence of the two things underlined limits power of the SB spell for this purpose. The first limits how often you can use it for this and the second limits how often it will be successful. The inverse relationship between the two drives the overall probability of successfully using SB lower than the product of these two individual conditions.
Assuming you have everything in place to use SB when the first saving throw is rolled - If the DC is high chances are you will not get to cast SB. If the DC is low chances are the enemy will make the reroll when you do get to cast it.
For example if everything is in place and you have a 6th level wizard with 6 higher level slots:
DC is 4 - your chance of successfully using SB to flip an individual save is 16%. Meaning if you do everything to put yourself in the position to use it 6 times a day against a DC 4, SB will flip a save once on average
DC is 11 - your chance of successfully using SB to flip an individual save is 25%. Meaning if you do everything to put yourself in position to use it 6 times a day SB will flip a save 1.5 times on average. This is as good as it gets for this purpose.
DC is 17 - your chance to successfully use SB to flip an individual save is 16%. Meaning if you do everything to put yourself in position to use it 6 times a day SB will flip a save once on average.
That presumes you have done everything else - you moved in close enough, you cast the higher level slot, you have SB and saved a slot for it etc. Regardless of DC, over 75% of the time you will have done all that but there will be no additional affect from SB (advantage to an ally not withstanding).
