I'm looking at the Tavern Brawler Feat and realizing that it is likely going to drop in effectiveness as a character levels up and encounters more monsters resistant/immune to non-magical attacks.
Are there ways for a pure fighter to be able to make an improvised weapon attack magical for the means of bypassing resistance/immunity?
The description of the Battle Standard of Infernal Power (a very rare magic item) from the Baldur's Gate: Descent into Avernus adventure (p. 223) includes the following benefit:
While you hold the banner, your weapon attacks and those of all allied creatures within 300 feet of you count as magical for the purposes of overcoming damage immunities and resistances.
So if you, or one of your allies, is holding the banner your weapon attacks count as magical, even the improvised weapon attack you make with an old frying pan or dead goblin imp.
From the Monster Manual section on "Vulnerabilities, Resistances, and Immunities" (p. 8; emphasis mine; note errata):
a magical attack is an attack delivered by a spell, a magic item, or another magical source
This doesn't require the magic item to be a weapon (normally), so if you hit someone with, say, a Broom of Flying, it is an improvised weapon attack and it is magical.
There is some disagreement whether the above works (see here and here at your own risk), arguing that magic items don't count as magical weapons when used as improvised weapons. This I agree with; however, the resistance is against nonmagical attacks and, by the above quote, an attack is magical when delivered by a magic item.
Regardless of the rules-quarreling, I need only imagine a burly fighter wail at a golem with a Broom of Flying to know that attacks delivered through the medium of an enchanted broom is in fact, magical (in more ways than one).
To make a "magical attack", you have to meet one of three conditions laid out by the DMG:
... a magical attack is an attack delivered by a spell, a magic item, or another magical source
A fighter using an improvised weapon would not be a spell, so thats one condition we can cross off.
A improvised item is potentially a magic item, but it is not necessarily a magic weapon. Jeremy Crawford has stated:
A magic shield is not a magic weapon, unless its text says otherwise.
The PHB describes an improvised weapon as the following:
Sometimes characters don’t have their Weapons and have to Attack with whatever is at hand. An Improvised Weapon includes any object you can wield in one or two hands, such as broken glass, a table leg, a frying pan, a wagon wheel, or a dead Goblin.
Often, an Improvised Weapon is similar to an actual weapon and can be treated as such. For example, a table leg is akin to a club. At the GM’s option, a character proficient with a weapon can use a similar object as if it were that weapon and use his or her Proficiency Bonus.
Being considered magical is not the same as being a magical weapon. A magic item does not become a magical improvised weapon when used in that manner.. This particular question has been asked many times.
For the last condition, we'd have to analyze the fighter's kit to see if they are a "magical source". I've looked through most of the archetypes and do not see that any of them grant the ability to turn a mundane object into a magic weapon.
Note: I've specifically had to look up the Sage Advice to see if a Eldritch Knight's bonded weapon was magic, and it isn't. Nothing else would come close to achieving this.
So to summarize, it does not appear a fighter has any way to make a magical attack with an improvised weapon..
In order to make a "magical attack", you have to meet these conditions:
a magical attack is an attack delivered by a spell, a magic item, or another magical source
Is a magic item a magic weapon or an improvised weapon? Jeremy Crawford has stated:
A magic shield is not a magic weapon, unless its text says otherwise.
That says nothing about being a magic “improvised” weapon or not. It may be the same to him but either way it is unofficial or RAI at best.
The PHB describes an improvised weapon as the following:
Sometimes characters don’t have their Weapons and have to Attack with whatever is at hand. An Improvised Weapon includes any object you can wield in one or two hands, such as broken glass, a table leg, a frying pan, a wagon wheel, or a dead Goblin.
A set of Sending Stones in the hand would be in line with the Improvised portion of this. Sending Stones being used to hit someone in the head would be the same as regular rocks. Would this make it magical attacks? Yes, based on the description of magical attacks laid out earlier. Sending stones are a magic item and can be improvised.
As it concerns overcoming resistance, the text on monster stat blocks state:
resistance to bludgeoning, piercing and slashing from nonmagical attacks.
This does not limit the attack to just weapon attacks.
In conclusion
Using a magic item as an improvised weapon laid out by the rules would overcome resistances.
Note: Sending Stones are being used as an example and there are many other magic items that would apply.
Two fighter subclasses can make an improvised weapon magical.
The arcane archer can, at 7th level, magic any non-magical arrow fired from a shortbow or longbow magical. An improvised arrow or short or long bow, then, can be affected in this manner. This may be disappointing, since it only works with weapons that the DM rules are similar enough to a bow, which means that most of the utility of improvised items are lost here anyways.
The Edritch knight can, at 7th level, cast 2nd level spells. This means they can cast Magic Weapon, which they can use to make an improvised weapon magical. They can do this with a second level spell slot for up to 10 concentration-requiring minutes. This can work on any nonmagical weapon, implicitly including improvised weapons, but can only be done in very limited quantities and may fail at any time when you are hit.
