Are there any abilities or magic items that allow weapon attacks to ignore damage immunity?

Question

DnDBeyond recently released a League of Legends themed adventure that introduces new unofficial subclasses that, while certainly interesting, pose some serious balance ramifications.

One of the biggest offending abilities allows a PC's weapon attacks to ignore resistances and immunities for any damage inflicted by the weapon attack. I can't think of single feature or item that grants this effect.

Are there any features or magic items that allow weapon attacks to ignore damage immunity against the weapon's damage type?

This effect can be permanent or transient.

Answer 1

The Sword of Answering

The Sword of Answering (Dungeon Master's Guide, pg. 206) has a special ability which ignores immunity and resistance (emphasis mine):

You gain a +3 bonus to attack and damage rolls made with this sword. In addition, while you hold the sword, you can use your reaction to make one melee attack with it against any creature in your reach that deals damage to you. You have advantage on the attack roll, and any damage dealt with this special attack ignores any damage immunity or resistance the target has.

Answer 2

Most resistances and immunities have a way to bypass them already.

In many cases a way of bypassing a resistance or immunity is already defined in the resistance or immunity. Resistance against bludgeoning piercing and slashing damage is usually restricted to nonmagic weapons. Sometimes it is further restricted to nonmagic or non-adamantine weapons (e.g. Devils, Lycanthropes, Gargoyle). Getting magic attacks or silver weapons and so on will bypass the feature.

There are few creatures with resistance or immunity against bludgeoning, piercing, and / or slashing without such a bypassing mechanism in the MM: Resistance: demilich, treant, awakened tree, swarms (6 instances), Immunity: Black Pudding, Ochre Jelly

There is the vorpal sword.

The vorpal sword (DMG p. 209) states:

You gain a +3 bonus to attack and damage rolls made with this magic weapon. In addition, the weapon ignores resistance to slashing damage.

There is not a balancing problem per se.

Some classes already have features that make their attack magical, such as the artificer's Infuse Item, the monk's ki-empowered strike, and the the warlock's pact weapons. These will bypass most instances of resistances and immunities as stated above.

Furthermore, the feature of bypassing resistances is very situational. It will do nothing when the monsters do not have such resistances / immunities. Compare that to other class features which will more consistently increase damage such as the Barbarian's Rage, or the ranger's Colossus Slayer. Those will generally deal more additional over average combats against different types of opponents. It is true that the damage is potentially drastically increased against the right kind of opponents. But similar features (in that way) exist such as the ranger's Favored Enemy. In the end, the impact of the feature strictly depends on the opponents that are presented in the campaign.

Answer 3

Ki-Empowered Strikes for the monk effectively does nearly the same thing for their unarmed strikes- nothing is generally immune to bludgeoning damage, and their unarmed strikes counting as magical bypasses all the various nonmagical bludgeoning immunities/resistances. There are a couple resistances that Ki-Empowered Strikes doesn't bypass- mainly swarms and a few plants.

Having a weapon start counting as magical (via spells similar to magic weapon or obtaining an actual magic weapon) has mostly the same effect of bypassing the majority of existing monsters' resistances/immunities for any class that uses them.

You can find lists that cover most monsters that would have a meaningful difference between 'you have a magic bludgeoning/slashing/piercing weapon' and 'you ignore resistances/immunity entirely with your bludgeoning/slashing/piercing weapon' here (for resistances) and here (for immunities). To summarize:

• Oozes/pudding/slimes are generally slashing immune, magical or not.
• Several plants have a general resistance to bludgeoning and/or piercing.
• There are ~10-15 adventure-specific monsters that have general resistances.
• Swarms tend to have a general resistance to bludgeoning/slashing/piercing, magical or not.

To put it another way, based on my D&DB search results, having a weapon become magical bypasses about ~315 monsters' resistances and/or immunities, and ignoring resistances/immunities entirely bypasses an additional ~45 monsters on top of that.

If we narrow that to just piercing damage, which is the main concern with a class feature that allows guns to bypass resistances/immunities, that second number goes down to ~30, all of which are resistances.

So, to answer the question of:

Are there any features or magic items that allow weapon attacks to ignore damage immunity against the weapon's damage type?

Yes, making a bludgeoning or piercing weapon magical effectively makes no pre-existing monsters immune to that damage. Slashing has a few exceptions.