Is there any way to actually impose disadvantage (not just cancel advantage) on saving throws against spells for a creature with Magic Resistance?

Question

Many creatures in D&D 5e have a feature called Magic Resistance. One such is the Archmage:

Magic Resistance. The archmage has advantage on saving throws against spells and other magical effects.

So the Archmage has advantage on saving throws against magical effects. Suppose I wanted to weaken the Archmage's resistance to magic - I use some effect that gives disadvantage on the saving throw for my spell. Normally, this would balance out to a straight roll, as the rules for advantage and disadvantage say:

If circumstances cause a roll to have both advantage and disadvantage, you are considered to have neither of them, and you roll one d20. This is true even if multiple circumstances impose disadvantage and only one grants advantage or vice versa. In such a situation, you have neither advantage nor disadvantage.

This seems to set a baseline for a straight roll on saves against magical effects, that the Archmage can never roll at disadvantage against magical effects.

But is there a way to get around this? Keep in mind, such an ability must respect the specific beats general rule:

That said, many racial traits, class features, spells, magic items, monster abilities, and other game elements break the general rules in some way, creating an exception to how the rest of the game works. Remember this: If a specific rule contradicts a general rule, the specific rule wins.

The general rule here is that advantage and disadvantage balance out to a single die roll, so getting around Magic Resistance must either specifically override the rule for advantage and disadvantage, or eliminate Magic Resistance entirely.

Is there any way (e.g. magic item, class feature, spell, etc.) to force a creature with Magic Resistance to make a save against a magical effect with disadvantage?

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While writing this up I did find this closed question which asks generally how to combat creatures with Magical Resistance, I intend this to be a (hopefully) more focused version of that question.

Answer 1

To my knowledge there is no feature that lets you give super-disadvantage and override a present advantage. So the only option is to take away the Magic Resistance feaure. While there is no specific way for that, there is:

Polymorph

The 4th level spell polymorph and its big brother true polymorph are able to completely overwrite the statblock of a creature. It is replaced by that of a beast. Since the Magic Resistance feature is part of the statblock, that is lost as well.

While they do get to save againts the transformation, they only roll 1 save, and once it sticks their saves will not only be without advantage, but their modifiers are also likely in the gutter as well!

You might have a bit of trouble targeting them with other effects as they are now a beast and not what they were before, but effects that could affect both will persist even when polymorph wears off.

You also cannot damage them properly, but in exchange they are helpless as well.

Answer 2

(This answer appears to be invalidated by 2018 PHB erratum that nerfed Portent to only replace 1 die, see How does the Diviner's Portent ability interact with Advantage and Disadvantage?, thanks @NautArch for pointing that out.)

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A divination wizard's Portent ability can be used on enemy saves, replacing their d20 roll regardless of advantage or disadvantage.

Starting at 2nd level when you choose this school, glimpses of the future begin to press in on your awareness. When you finish a long rest, roll two d20s and record the numbers rolled. You can replace any attack roll, saving throw, or ability check made by you or a creature that you can see with one of these foretelling rolls. You must choose to do so before the roll, and you can replace a roll in this way only once per turn.

This replaces the d20 (straight / advantage / disadvantage) roll (from a tweet by Jeremy Crawford), before the creature's modifiers are added. So it can be combined with other additive modifiers like Bane (minus 1d4).

It's definitely not disadvantage, but posting anyway as requested by the OP. Using on enemy saves against save-or-suck spells is one of the most effective combat uses for low d20 portent rolls. (If you're curious how this plays in practice and enjoy watching live-play D&D videos, a character in the Fantasy High series on youtube makes good use of her low and high portent rolls this way, especially in season 2 when they're higher level and have more important save-or-suck spells.)

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Also note that the Lucky feat does not work for this. It works for Attack rolls against you, but never checks/saves by other creatures. (This question quotes the wording; sage advice confirms it can turn advantage into super-disadvantage for attacks against you, although without removing mechanical benefits of advantage such as Sneak Attack.)

Answer 3

Because of the advantage/disadvantage rule. There is no easy way to force a roll with disadvantage.

You have to remove the advantage somehow.

The only way I am aware of is to strip the target of their resistance, such as the spell “polymorph” which itself would be subject to the protection. But while polymorphed, it would be susceptible to any other magic effect, which would carry over after polymorph ended.

Otherwise you could also Wish resistance away.. Although that is a bit extreme, it would grant your proper advantage roll.

The other theoretical options would be to change the saving throw to be non-magical somehow. Or a “ignore spell resistance” ability added. Keep in mind such abilities, which negate creatures benefits, are very rare, examples might be the Gloomstalker’s “anti-darkvision” ability. Or the Elemental Adept feat ability to ignore damage resistance. Exceptions to exceptions can get hard to follow for rules arbitration.

Combining the theoretical ideas with the actionable “wish”. You could wish to be able to ignore magical resistance, or for your spells to be considered non-magical (my head hurts).

Answer 4

You could use a luck point which is almost disadvantage, since you are making them roll twice and use the lower.