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?
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.