Some arcane casters make use of a specific kind of Spellcasting Focus known as an arcane focus in order to be able to cast specific kinds of spells.
There are three different paths that a spellcaster might take in order to cast an arcane spell that requires a material (M) component.
They can make use of the spell components, a component bag, or a spellcasting concentration.
All three options are available to them.
It is recommended that one utilize either the bag or the focus in order to make casting spells as simple and straightforward as humanly possible.
A crystal, ball, rod, staff, or wand are all examples of the many possible shapes that an arcane focus might take.
The costs associated with these focuses may be found in Chapter 5 of the Player’s Handbook.
Staff with an arcane concentration that costs 5 gold pieces is not, strictly speaking, the same thing as a quarterstaff (cost 2 sp).
It is possible to use a quarterstaff with an arcane focus staff, but it is not possible to use a conventional quarterstaff with an arcane focus staff. Therefore, as you are developing your characters, it is important to keep this in mind.
It’s possible that your DM will let you employ other things outside an arcane focus as well.
Bards, Sorcerers, Warlocks, Wizards, Artificers, Eldritch Knights (a Fighter subclass), and Arcane Tricksters are all examples of arcane casters in D&D 5e.
Other examples include Warlocks and Artificers (Rogue subclass).
However, for whatever (utterly incomprehensible) reason, the rules stipulate that bards, artificers, arcane tricksters, and eldritch knights are not permitted to make use of an arcane concentration.
Fortunately, all of these classes have the ability to cast their spells by making use of a component bag.
Alternately, some of these spellcasters have access to a wider variety of foci for their spellcasting.
For instance, a bard may utilize a musical instrument as a focus for their spellcasting, but an artificer may employ items often associated with thieves or with artisans.
It is also important to remember that you must have a free hand in order to use an arcane focus or component bag when casting spells.
Because of this, certain characters might want to consider switching their primary concentration when it comes to spellcasting.
For instance, in order to perform a spell that requires S or M components, an eldritch knight who is wielding both a sword and a shield will have to put down their weapon.
Fortunately, the knight may use their free ability to Interact with an Object to retrieve the sword after it has been dropped.
There are also magical artifacts that have the potential to serve as a spellcasting focus, giving your character the ability to utilize both hands at once.
For instance, a wizard can utilize the Hat of Wizardry, which is a common magic item, as a spellcasting focus.
Similarly, the Ruby of the War Mage, which is also common magic, allows you to use a weapon as a spellcasting focus in exchange for one of your attunement slots.
If a character has the War Caster feat, even if they are holding a weapon or shield in their hands, they are still able to cast spells that have a somatic (S) component.
- An orb
- A crystal
- A specially constructed staff
- A wand-like length of wood
What is allowed to be an Arcane focus?
Arcane Focus is a Specific Item
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.
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 component pouch
- it’s listed as a common item, implying that it’s not considered too much of a big deal
- it requires attunement