D&D 5e establishes a pretty decent divide between physical and magic combatants. In particular, their core damage mechanic is split in two: physical attackers use weapons to deal damage, magical attackers use cantrips (and spells) to deal damage.
This leaves an underserved demographic in the gap: characters that want to be primarily physical (taking one of the physical-oriented classes, like Fighter or Rogue) but with a mystical twist to how they attack, slinging fire or whatnot. A few cantrips kinda serve this: Shillelagh does the opposite, kinda, letting a mage class use a physical weapon with their casting stat; the "bladetrips" are bit closer to this, but they're still cantrips, not weapons, and so can't be used with features like Sneak Attack or Rage, and are balanced on the cantrip scale (increasing at levels 5/11/17).
So, let's define something that actually works!
Arcane Weapons are a new class of weapons that deal damage thru magic. These can take many forms, from rune-inscribed blades to magic wands to arcane blasters.
Rather than listing an explicit and limited set of Arcane weapons, Arcane weapons can be produced by starting from any Martial weapon's stats, and choosing a magical damage type to use for it instead, making appropriate modifications based on the damage type. The GM and player should then work together to decide on the actual physical form of the weapon.
- If you choose Magical Slashing, Magical Bludgeoning, Magical Piercing, or Poison, the damage is unchanged.
- If you choose Fire, Ice, Lightning, or Acid, the damage is reduced by one step.
- If you choose Radiant, Necrotic, Psychic, Thunder, or Force, the damage is reduced by two steps.
Damage "steps" are: (1d12 or 2d6) => 1d10 => (1d8 or 2d4) => 1d6 => 1d4 => 1 => 1 => .... If you move to a step with a choice of damage dice, you can choose either option.
Alternately, one can start from a Simple weapon, and ignore one damage reduction step. For example, a Needler Wand produced from Light Crossbow stats (1d8 damage) switched to ice damage would still deal 1d8 damage on hit, but switched to radiant would deal 1d6 damage on hit. (This does not make the Arcane weapon a Simple weapon, it just expands the variety of weapon stats one can use, particular in ranged weapons.)
Wielding an Arcane weapon uses all the same rules as the corresponding weapon it's based on. For example, if you start from Greatsword stats to produce an Immolation Staff that deals fire damage, it's still a Two-handed Heavy Melee weapon, that happens to deal 1d10 fire damage on hit instead of 2d6 slashing. As another example, if you start from Light Crossbow stats to produce a Needler Wand that deals ice damage, it's still a Two-handed Loading Ranged weapon with an 80/320 range, that happens to deal 1d8 ice damage instead of 1d8 piercing.
Abilities that depend on or otherwise reference Martial weapons can also be used with Arcane weapons based off the specified weapon types. For example, Monks can choose any Arcane weapon they have proficiency in as one of their monk weapons.
However, no character has proficiency with Arcane weapons by default; they can gain proficiency in various methods, as described below.
Arcane Weapon Proficiency
Arcane weapons require special proficiency to use. Without this proficiency, an Arcane weapon can't be used at all; it's simply a magical stick or what-have-you.
Whenever a source would grant a player proficiency with one or more martial weapons (such as the first level of most martial classes, or some heritage choices such as Elf) you may choose to instead gain proficiency in Arcane weapons using the stats of any of those weapons. This is an all-or-nothing choice; all of the gained Martial weapon proficiencies are switched over to Arcane equivalents, or none are.
That is, a class that grants proficiency with all Martial weapons, like a Fighter, can instead choose to gain proficiency with all Arcane weapons based on Martial weapon stats. A class with limited proficiencies, like the Monk who receives only shortsword proficiency, can instead choose to gain proficiency with Arcane weapons based on shortsword stats.
(A player could use multiclassing to easily get both Martial and Arcane proficiencies, such as taking a level of Fighter and a level of Barbarian, if they so chose.)
Any class that gains cantrips can choose to replace one known cantrip with proficiency in all Arcane weapons instead.
Unarmed strikes do not have an Arcane equivalent in general, but Monks and similar classes that can make their unarmed strikes magical have additional options; see below for details.
The Weapon Master feat can, as any of your four chosen proficiencies, instead grant proficiency with Arcane weapons based off the chosen weapon's stats.
If you're proficient in at least one Arcane weapon, or can cast at least one spell, you're automatically proficient in all Arcane weapons based off of Simple weapons you're proficient in, as well.
When you gain proficiency with at least one Arcane weapon, choose a magical damage type to be your primary damage type. Whenever you attack with an Arcane weapon, you may choose to have it deal your primary damage type instead of whatever damage type it would normally deal, applying the same modifications to the base weapon's damage dice that you would if designing the weapon from scratch.
As well, when you gain proficiency with at least one Arcane weapon, if you don't already have a spellcasting ability from another source, choose Int, Wis, or Cha as your spellcasting ability. Whenever you attack with an Arcane weapon, you may choose to use your spellcasting ability instead of Str/Dex for attack and damage rolls using the weapon.
For example, if a character chooses Int and Psychic as their casting stat and primary damage type, then wields the Immolation Staff described above (greatsword switched to fire damage), they can choose to add either their Str or Int to attack and damage, and can choose to deal either 1d10 fire damage or 1d8 psychic damage on a hit.
If the same character wielded the Needler Wand (light crossbow switched to Ice damage), they can choose to add either their Dex or Int to attack and damage, and can choose to deal either 1d8 ice damage or 1d6 psychic damage on a hit.
Arcane Unarmed Strikes
Replace the Monk's 6th level Ki-Empowered Strikes ability with:
You gain the ability to make your unarmed strikes into Arcane Strikes. If you do not already have a spellcasting ability and/or an Arcane primary damage type, choose them as if you have just gained proficiency in an Arcane weapon.
Any time you make an unarmed strike, or attack with a monk weapon and replace its damage with your unarmed strike damage, you can choose to use your spellcasting ability instead of Str or Dex, and you can choose to replace its damage type with your Arcane primary damage type. If your chosen Arcane primary damage type would normally reduce a Martial weapon's damage die by two steps, your unarmed strike damage is reduced by one step when you replace its damage type (it is otherwise unaltered).
Your unarmed strike now counts as an Arcane weapon (for the purpose of applying Fighting Styles/etc).
For example, a Monk could choose Wis and Fire for their unarmed strike, allowing them to add their Str, Dex, or Wis to attack and damage for unarmed strikes, and deal Bludgeoning or Fire damage equal to their normal unarmed strike damage. If they are wielding a monk weapon and choose to replace its damage with their unarmed strike damage, they can also switch its damage type to Fire.
If they instead chose Radiant for their damage type, they can still freely add their Str, Dex, or Wis to their attacks, and can choose to either deal their ordinary amount of Bludgeoning damage, or deal Radiant damage reduced by one step.
If the Fire Monk from above wields a Radiant arcane weapon that they've chosen as a monk weapon, they can choose to have it deal Radiant or Fire damage, and its damage is not reduced in either case.
Arcane Fighting Styles
These additional fighting styles are available to any class, and all require that you have proficiency with at least one Arcane weapon.
- Arcane Fire Style: Whenever you wield an Arcane weapon dealing fire damage, increase the damage die by one step.
- Arcane Ice Style: Whenever you hit an opponent with an Arcane weapon dealing ice damage, their speed is reduced by 10ft until the start of your next turn unless they're immune to ice damage. This effect only applies once per opponent; it does not stack if an opponent is hit multiple times.
- Arcane Acid Style: Whenever you hit an opponent with an Arcane weapon dealing acid damage, you may deal the weapon's base damage to another target within 5ft of the original target.
- Arcane Lightning Style: Whenever you hit an opponent with an Arcane weapon dealing lightning damage, you may spend your reaction to deal additional lightning damage equal to the weapon's base damage.
- Arcane Poison Style: Whenever you hit an opponent with an Arcane weapon dealing poison damage, they have disadvantage on the next saving throw they make before the start of your next turn unless they're immune to poison damage or the poisoned condition.
- Arcane Radiant Style: Whenever you wield an Arcane weapon dealing radiant damage, your attacks with that weapon ignore half cover and three-quarters cover.
- Arcane Necrotic Style: Whenever you hit an opponent with an Arcane weapon dealing necrotic damage, increase the damage die by two steps if the opponent is missing any of its hit points.
- Arcane Psychic Style: Whenever you hit an opponent with an Arcane weapon dealing psychic damage, they have disadvantage on the next attack roll they make before the start of your next turn unless they are immune to psychic damage.
- Arcane Thunder Style: Whenever you hit an opponent with an Arcane weapon dealing thunder damage, you may deal the weapon's base damage to every target within 5ft of the opponent besides you, but your attack makes a thunderous noise that can be heard up to 100ft away.
- Arcane Force Style: Whenever you hit an opponent with an Arcane weapon dealing force damage, you may choose to push them 10ft away from you in a straight line unless they are immune to force damage.
Arcane Weapon Strike
While arcane weapons interact well with martial classes' Extra Attack features, they don't compare well with cantrips for spellcaster classes that never get extra attacks. Green Flame Blade and similar bladetrips let casters flip a weapon into cantrip scaling, so let's run on that same principle:
Arcane Weapon Strike
Casting Time: 1 action
Range: Self (5-foot radius)
Components: S, M (an arcane weapon worth at least 1 sp)
You brandish the arcane weapon used in the spell's casting and make an attack with it as normal. On a hit, the target suffers the weapon attack's normal effects, and your magic flares thru the weapon to strike another target you can see within 5 feet of the original target. The second creature takes damage equal to your spellcasting ability modifier, and of the same type as the arcane weapon.
This spell's damage increases when you reach certain levels. At 5th level, the damage to both targets increases by 1d10/1d8/1d6 (depending on arcane damage type) for melee attacks, or 1d8/1d6/1d4 for ranged attacks. They increase by another die at 11th and 17th level.