If a creature has advantage against spells and magical effects, is it resistant to magical attacks?

Question

My DM said that monsters that have advantage against spells and magical effects are extra resistant to damage.

We fought some demons that have resistance to slashing, piercing, and bludgeoning damage from non-magical attacks. We all have magical weapons, but he was saying that the monsters were still resistant to the damage because they have advantage against magical effects.

Is this a correct interpretation of the rules?

Answer 1

Resistance and advantage are two separate mechanics

Advantage has to do with rolling the dice, resistance has to do with reducing damage in half.

Easy illustrative example is a Dwarf

Dwarven Resilience. You have advantage on saving throws against poison, and you have resistance against poison damage (explained in chapter 9). {Basic Rules, p. 15}

The dwarf rolls 2d20 whenever she has to make a saving throw against poison. Because a dwarf is resistant to poison, any poison damage she takes is halved.

Resistance

If a creature or an object has resistance to a damage type, damage of that type is halved against it. {Chapter 9}

Advantage and Disadvantage

... ability check, attack roll, or saving throw is modified by special situations called advantage and disadvantage. ... When you have either advantage or disadvantage, you roll a second d20 when you make the roll. Use the higher of the two rolls if you have advantage, and use the lower roll if you have disadvantage. {Basic Rules, p. 5}

What to share with your DM from the rules

A creature with advantage on saving throws versus magic/spells still rolls the saving throw: 2d20, pick the higher score. Resistance is applied separately once the amount of damage is determined by the saving throw's success or failure.

Resistance and then vulnerability are applied after all other modifiers to damage.
{Basic Rules, p. 78}

Dwarf Example:

Exceka the dwarf is hit by a giant scorpion's tail; the damage roll for poison is 24. Exceka has advantage on the saving throw. She rolls a 3 and a 19, she saves. Damage is halved, so she takes 12 damage, except, she has resistance to poison! She takes half of that damage: 6 HP is the final damage.
_{FWIW, the wizard abjuration school has a feature that grants both resistance to spell damage and advantage on spell saves. If one of them implied the other, it would be redundant and unnecessary to specify both. Thank you, @Ryan C. Thompson}

Demon Example: Balor

What Magic Resistance means for this demon is explicitly spelled out in its stat block.

Magic Resistance: The balor has advantage on saving throws against spells and other magical effects. (MM, p. 55)

That is different from the Damage Resistances in the stat block (MM p. 55) that says

Damage Resistances Cold, lightning, bludgeoning, piercing, and slashing from non magical weapons

And that is different from

Damage Immunities Fire, Poison

And that is different from

Condition Immunities Poisoned

To illustrate: Balor is hit by Ice Storm spell (cold + bludgeoning damage)

Each creature in the cylind⁠er must make a Dexterity saving throw. A creature takes 2d8 bludgeoning damage and 4d6 cold damage on a failed save, or half as much damage on a successful one.

A Sorcerer cast the spell, and rolled damage: 12 bludgeoning and 16 cold.

1. Step 0. Is the spell a magical effect? Yes. Apply advantage to the saving throw.

2. Step 1. Is there a saving throw? Let's say the Balor rolls a 9 and a 17; spell caster's DC is 16. Balor saves.

3. Step 2: Save -> half damage.
   a. The magical bludgeoning damage becomes 6. Demon is not resistant to magical bludgeoning attacks. (The barbarian with the non-magical maul who did 16 damage last time only did 8, due to Damage Resistance cited above)

b. The cold damage becomes 8.

1. Step 3: Now apply resistance.

Balor is resistant to cold, so it takes only 4 points of the 8 cold damage.

Final note

Damage is by type. Magical is not a damage type (but it is a kind of attack or effect). The damage types are on basic rules p. 78, and in PHB p. 196:

Acid, Bludgeoning, Cold, Fire, Lightning, Necrotic, Piercing, Poison, Psychic, Radiant, Slashing, Thunder

Answer 2

Agreed with Korvins answer, advantage just means roll two dice and use the higher number, that means if they are doing a spell save roll against your magic they have a better chance of avoiding it maybe, but they don't take half damage off the bat like with resistances as that's different.

If you tried to hit some one with a fireball for example they might have to do a dexterity roll to avoid it, they could do that roll with advantage which gives them a better chance.
If they fail however and take damage from it you THEN look at if they are resistant to magic or specifically fire magic and if they are then can take reduced damage, however the advantage doesn't cause changes in damage itself.

Answer 3

Your DM is confusing advantage and resistance. They're not the same, and they don't stack multiplicatively, because they don't even apply to the same things.

Advantage is always done on a d20 roll such as an attack roll, saving throw, ability check, etc. If a fire-damage spell doesn't allow a save at all, then your advantage on saving throws vs. fire-damage spells does nothing against that specific spell.

Resistance applies strictly to damage. The exact and full list of damage types is found in PHB page 178. If an ice spell deals effects that are all written as cold-themed (aka "fluff") penalties (aka "actual mechanics"), but that spell deals no actual cold damage, then your resistance to cold damage does nothing against that specific spell.

Thus, "resistance to slashing, piercing, and bludgeoning damage from non-magical attacks" always refers specifically to this list of damage types. As you can see here, slashing damage from actually magical weapons is not covered.

But the demons also have advantage vs spells and magical effects. In other words: Advantage on their own rolls (mainly saving throws here) caused by spells and magical effects. Advantage is on rolls. There is no such thing as "advantage vs. damage".
If the DM insists it is so and that advantage should somehow be applied to your damage, then roll damage twice and keep the highest value. After all that is how advantage actually works on d20 rolls, right? And it would be ridiculous, right?

When you strike with your magical sword, the demon doesn't get to roll anything to be able to have the right to apply advantage on such a non-existent roll. And the DM sure isn't gonna apply advantage on your attack roll against that demon!

Sure, your sword attack is magical - but there is no roll by the demon in the first place for it to apply advantage to.
Your DM conflated that to mean that the advantage somehow magically transforms into resistance instead. Your DM is incorrect.

Now, if your magical sword caused paralysis on a hit, with a saving throw for the target, then the demon would gain advantage on the paralysis magical effect. But even if the base sword attack/damage itself were a "magical effect" (which it's not - it's merely direct weapon damage), that still doesn't grant the demon any roll, so it's got no way to apply its advantage... against the strike itself. You make the attack and damage rolls, not the demon.

Now, a DM can change anything he wants and give creatures resistance to magical physical weapons too. (Though I'd recommend against this; it unfairly hurts only martial characters. The spellcasting classes don't need any more advantages compared to the poor martials, especially at higher levels because the "linear fighters, quadratic wizards" syndrome still exists somewhat in 5e).

But if your DM trying to follow the rules as written, then he made a mistake in this instance.