What is allowed to be an Arcane focus?
Reading the PHB chapter 5 equipment pg. 151. It describes an arcane focus as such:
Arcane Focus. An arcane focus is a special item — an orb, a crystal, a rod, a specially constructed staff, a wand like length of wood, or some similar item — designed to channel the power of arcane spells. A sorcerer, warlock, or wizard can use such an item as a spellcasting focus, as described in chapter 10.
Now I have been coming up with a dwarven war mage character concept, and was curious to know would it be allowed to have an arcane focus as warhammer?
Arcane Focus is a Specific Item
The arcane focus is found in the Adventuring Gear section of the equipment chapter (PH page 150, SRD page 69, SRD link). Your options for arcane focus are:
So no, you cannot use a warhammer as a focus.
An arcane focus is "…an orb, a crystal, a rod, a specially constructed staff, a wand-like length of wood, or some similar item…." (emphasis mine). A warhammer or other weapon is not similar to any of the listed items.
Can A DM allow it? You Bet
That being said, if your DM says it's OK, go for it. Paladins and clerics can use a holy symbol as a focus, and that can be "…carefully engraved or inlaid as an emblem on a shield…." Using that as precedence, you can thusly allow arcane magic users to use the same rules.
Just know that in so doing, you are giving the arcane magic users an ability they didn't have before and devaluing the divine caster's unique schtik.
1 An exception that requires a common magic item
- If the warhammer has a Ruby of the War Mage imbedded in it. The Ruby of the War Mage (Xanathar's Guide to Everything) is a magical gem that can be attached to a weapon, at which point that weapon can be used as a spellcasting focus. Things to note
- it says "spellcasting focus", not "arcane" or "divine", implying that it works with all spellcasting in the same way as a comnponents pouch
- it's listed as a common item, implying that it's not considered too much of a big deal
- it requires attunement
College of Swords Bards can use a weapon as a spellcasting focus as well
The Ruby of the War Mage is listed in Xanathar's Guide to Everything; it's a magical gem that can be attached to a weapon, at which point that weapon can be used as a spellcasting focus. Things to note: it says "spellcasting focus", not "arcane" or "divine", implying that it works with *all* spellcasting in the same way as a comnponents pouch; it's listed as a common item, implying that it's not considered too much of a big deal; it requires attunement, implying that there are some balancing concerns to allowing casters to wield weapons without penalty.
The purpose of material components (which includes a spellcasting focus) is to control action economy.
To cast a spell you need a hand free to hold the components. To hit someone you need a hand free to hold the weapon. To defend yourself you need a hand free to hold the shield.
This means that there is a limit on how many different actions you can do; namely two (four, if you are a thri-keen).
If you allow one item to be used for two purposes then you are changing the action economy.
You also remove some of the consequences of choices. For example, if you end your turn with a sword and a shield in your hands then you can't cast a spell as a reaction. Your choice has a meaningful consequence.
A quarterstaff is 2sp. A staff focus (both the arcane one and the druidic one) is 5gp. This suggests that a focus is an ornate, crafted item, made from expensive materials (in other words, not just a big stick). I imagine that if you hit someone with your focus it will damage the item, possibly making it unsuitable to be a focus.
Inscribing a holy symbol on a shield costs 5gp, half the price of the original shield.
The 12 different types of focus are listed in the Adventuring Gear section, not the Weapons section. The focus is called a "staff" or a "wooden staff", not a "quarterstaff". I think this clearly indicates that focuses are not supposed to be weapons.
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Nothing really says anything conclusive about that.
Jeremy Crawford answered this on twitter:
Jonh Rhyne: does an Eldritch Knight's bonded weapon count as an arcane focus?
Jeremy Crawford: It doesn't, but it wouldn't be unreasonable for a DM to allow it.
And Mike Mearls answered the following question this way:
criton volun: can a arcane focus be part of a 2 handed weapon or a single weapon , like a clerics holy focus can be part of a shield ?
Mike Mearls: I'd allow it
As you can see, using an arcane focus as weapon isn't really the issue, the thing is that neither confirmed but seem to agree that it is something reasonable for a DM to agree.
Arcane Focus. An arcane focus is a special item - an orb, a crystal, a rod, a specially constructed staff, a wand like length of wood, or some similar item - designed to channel the power of arcane spells.
While not quite similar to most foci, the quarterstaff and the warhammer are both categorized as Versatile Melee Weapons. You can claim that similarity but in the end, is up to the DM to accept it.
If my top priority is playing D&D RAW, I would rule against allowing a character to use a weapon as an arcane focus, because that is an enumerated benefit of at least one subclass. The entry for the bardic college of swords in Xanathar’s Guide to Everything includes “If you’re proficient with a simple or martial melee weapon, you can use it as a spellcasting focus for your bard spells.” That’s a third level feature of the subclass. As a DM, I’d be hesitant to give player characters subclass features from a different class for free.
Now that I’ve said that, my top priority as a DM isn’t always playing RAW. I don’t think it would be a game breaking thing to give a character. I’d probably be willing to tack it onto the war caster feat or the Martial/magic adept feats. And if the rest of the table is cool with him getting a marginally better action economy than RAW, it may make sense to just let him do it.
I understand using a class or subclass feature as a benchmark for how powerful allowing arcane foci as weapons would be; however, these examples are benchmarked for allowing *any* weapon (or most conveniently, a shield) to be an arcane focus. A quarterstaff is a downgrade from any weapon or shield. By allowing a quarterstaff to serve as an arcane focus, you're demanding the player pay upfront by trading attack power. A difference of +1 or +2 damage on every attack is closer in line to a class/subclass feature, or at least is a much smaller leap in game balance for a DM to rule it as allowed.