I was going to ask a question about which damage type is reduced by the "magical aura" referenced in the PHB under Damage Resistance and Vulnerability if the damage in question has multiple damage types, and I realised I didn't know of any such effect, perhaps rendering the question moot.
For example, a creature has resistance to bludgeoning damage and is hit by an attack that deals 25 bludgeoning damage. The creature is also within a magical aura that reduces all damage by 5. The 25 damage is first reduced by 5 and then halved, so the creature takes 10 damage. - (PHB 197)
Obviously this is a hypothetical situation purely to illustrate how a rule works, but it does suggest that there is an effect that would work in this way.
To illustrate the issue:
Barny, the monster, attacks Vera, dealing 5 bludgeoning damage and 7 fire damage.
Vera is in a -5 to damage aura, and has resistance to fire damage.
Do we reduce all of the bludgeoning damage, and then halve the fire damage? Do we reduce the fire damage by 5, and then halve it, leaving the bludgeoning damage completely alone? We can't apply the resistance first (see above) so how would we resolve this?
I can only think of Cutting Words, which says:
When a creature that you can see within 60 feet of you makes an attack roll, an ability check, or a damage roll, you can use your reaction to expend one of your uses of Bardic Inspiration, rolling a bardic Inspiration die and subtracting the number rolled from the creature's roll. - (PHB 54)
However, this applies to a specific damage roll. It would be up to the bard if we're reducing the fire damage or the bludgeoning damage, as they're always going to be separate rolls.
Is there a RAW (official content only, no homebrew) effect that reduces damage by a flat modifier (not a percent of the total damage), and that is indiscriminate of damage type, like the one suggested by this passage?