Is there a way to change the damage type being dealt by a weapon?

Last night I was playing as my level 20 Barbarian, when our party encountered an enemy who was immune to all damage, unless it was of a specific elemental type.

This was particularly unfortunate for me, since my Barbarian deals exclusively in piercing and slashing damage. This resulted in me effectively standing around acting as a damage sponge while the party’s spell-casters dealt all the damage.

While not a disastrous situation, it got me thinking:

Are there any ways to effectively change the type of damage I am dealing?

To be more specific, I am curious if there are any spells, enchantments, magics, magical weapons, etc which can change the type of damage a weapon’s normal attack would have dealt. For example, my Barbarian’s weapon deals slashing damage – I am wondering if there are ways to change the damage type into something else(eg into instead dealing fire or poison damage, or even piercing or bludgeoning).

I’ve read this similar question, but am playing 5e, not 3.5.

Is there any way to change falling damage to another damage type?

The rules on “Falling” state:

[…] At the end of a fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell, to a maximum of 20d6…

When looking at the reverse gravity spell I realized it states:

[…] If some solid object (such as a ceiling) is encountered in this fall, falling objects and creatures strike it just as they would during a normal downward fall…

And then in this related question “Do any damage resistances apply to Reverse Gravity?” it is shown that falling damage, even from this spell, is still just bludgeoning damage. Is there any way to change the type of damage that a fall inflicts?

Maybe some object explicitly has this property or there is a way to change all damage a creature takes to another type, which would thus include fall damage?

If this is possible, then certain creatures, like the treant, would take more damage from falling so pushing them off a cliff or using reverse gravity on them would be more effective.

If a method is available to both PC’s and Monsters, that would be ideal. But if there is a method only available to PC’s and another method only available to Monsters that would work as well.

How to determine the base weapon damage for weapons that exceed the size table?

So I am playing a barbarian wielding a Butchering Axe. Last night it happened that by some glory magic my human (medium sized) barbarian was being enlarged letting his terrifying weapon become even more devastating.

So we checked the rulebook for what its new base damage would be. But unfortunately, the table ends at medium sized 2d6 becoming 3d6.

But since the Butchering Axe at medium size has already a weapon damage of 3d6, we tried to make out the pattern of the table which at that point was ambiguous (for us at least) we thought it either might be 4d6 following the increase from 2d6 to 3d6, or it might be 3d10 due to the steps at lower base damage weapons.

So the question is:

What is the weapons damage to be rolled here and by which pattern would this be identified for weapons exceeding the books printed table?

Monster with resistance to bludgeoning and fire damage from nonmagical weapons have resistance to a warhammer’s bludgeoning but not its fire damage?

My friend has an enchanted silvered warhammer that grants it +1d6 fire damage. We’ll be fighting a fiend soon with resistance to fire and bludgeoning damage from nonmagical and non silvered weapons.

My friend’s warhammer makes half bludgeoning damage and full fire damage to the fiend?

Is damage taken when falling off a mount?

The mounted combat rules provide a few scenarios for falling off a mount:

If your mount is knocked prone, you can use your reaction to dismount it as it falls and land on your feet. Otherwise, you are dismounted and fall prone in a space within 5 feet it.

The rules for falling state:

A fall from a great height is one of the most common hazards facing an adventurer. At the end of a fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell, to a maximum of 20d6. The creature lands prone, unless it avoids taking damage from the fall.

In either circumstance of falling off your mount (falling prone, or using your reaction to land on your feet), do you take any damage from falling? Does it depend on the size of your mount?

Very related: It seems obvious that if you are on a flying mount at height, and fall off, you will take some damage – but the answer to the question above will probably determine where you measure the fall distance from in such a scenario (square above mount/top square of mount/bottom square of mount/square below mount).

Does the Heavy Armor Master feat reduce damage twice against a mixed damage attack?

Displacer Beasts have a Tentacle attack that does 7 (1d6 + 4) bludgeoning damage plus 3 (1d6) piercing damage. Does the bludgeoning and piercing damage reduction granted by Heavy Armor Master reduce that total damage by 6 or 3? If only 3, what determines which of the two damage types is reduced?

Expected damage of a moonblade with N runes?

As noted in David Coffron’s answer to another question, the moonblade (Dungeon Master’s Guide, pg. 217) has no fixed limit on how much damage it can do: there is no stated limit to the number of runes on the blade. It states:

A moonblade has one rune on its blade for each master it has served (typically 1d6 + 1). The first rune always grants a +1 bonus to attack and damage rolls made with this magic weapon. Each rune beyond the first grants the moonblade an additional property. The DM chooses each property or determines it randomly on the Moonblade Properties table.

Some of the items on the table include:

91–92: When you hit with an attack using the moonblade, the attack deals an extra 1d6 slashing damage.

93–94: When you hit a creature of a specific type (such as dragon, fiend, or undead) with the moonblade, the target takes an extra 1d6 damage of one of these types: acid, cold, fire, lightning, or thunder.

But as David notes in a comment, some runes have limits on how many times they may be gained. That means, once the moonblade has gained these runes, it cannot gain them again, and the odds of gaining the remaining runes—including the quoted damage-improving runes—improves in the future.

So then, assuming that each rune is determined randomly per the table, rerolling any rolls that result in a rune that would not stack with the runes already on the moonblade, and that the creature-type-specific runes are distributed evenly among the creature types (feel free to assume fractional runes for that purpose), what is the expected value of the damage roll of a moonblade with $ n$ runes? Assume that the hit has already taken place, so changes to the weapon’s accuracy, such as making it finessed, or that make it easier to target enemies in the first place, such as making it thrown, are irrelevant. Also, absent any information about the target, ignore the damage benefit that vorpal might have. The best answer would include the chance of a critical hit, including the possibility that the critical threat range has been doubled by the relevant rune.

Do not assume any GM intervention limiting the moonblade or otherwise making any particular ruling here: instead use a rules as written interpretation of the text.