What happens when I attack with an unidentified magic weapon? Assume I am proficient with the weapon, am capable of wielding it, and that the weapon does not require attunement.
The weapon is usable as a weapon
The wielder either gets the effect of the magic or they do not. The answer depends on the nature of the magic weapon.
Case 1: Flat Modifier (+1/2/3/etc..)
The wielder gets the effect
The DMG has a paragraph describing interactions between a character and magic items that might clue them into their properties or how to use them.
DMG page 136
Sometimes a magic item carries a clue to its properties. The command word to activate a ring might be etched in tiny letters inside it, or a feathered design might suggest that it's a ring of feather falling. Wearing or experimenting with an item can also offer hints about its properties. For example, if a character puts on a ring of jumping, you could say, "Your steps feel strangely springy." Perhaps the character then jumps up and down to see what happens. You then say the character jumps unexpectedly high.
A straight bonus to attack and/or damage of a magic weapon is present regardless if the character is aware of the bonus or not.
In the case of a weapon with no activation required, the magical bonus is activated simply in the course of using the object as it would normally be used.
Following the DMG guidance, a GM could offer additional description to a hit such as, "The blade strikes true, and slashes through the hide of the beast with surprisingly little effort."
Case 2: Passive Effect (e.g. mace of smiting)
The wielder gets the effect
The passive effect can be adjudicated in the same fashion as the flat modifier.
In the case of the mace of smiting, a clue to the effect could be in a similar fashion to the boots of jumping example in the DMG. The GM could add, "The mace connects with the golem with an unusually crisp ringing. The golem shudders as the strike seems to shake its entire body."
Case 3: Activation Required (e.g. dagger of venom)
The wielder cannot use the activated effect
The character must have sorted out how to activate the item before the activated effect can be used. The passive effects that do not require activation would still be applicable immediately. The DMG lists some options of how to do that in the section on identifying magical items. Briefly, the options are:
- Identify spell
- Focus on item for short rest
Case 4: Attunement Required
The wielder does not get the effect
In the case of attunement, none of the magical properties operate until the character is attuned as per the DMG:
Some magic items require a creature to form a bond with them before their magical properties can be used. This bond is called attunement...
Without becoming attuned to an item that requires attunement, a creature gains only its nonmagical benefits, unless its description states otherwise. For example, a magic Sshield that requires attunement provides the benefits of a normal shield to a creature not attuned to it, but none of its magical properties.
Standard +X items will utilize the bonus
DMG suggests player doesn't know
The DMG discusses Magical item knowledge with this paragraph (DMG, 136). Emphasis is mine:
Some magic items are indistinguishable from their nonmagical counterparts, whereas other magic items display their magical nature conspicuously. Whatever a magic item’s appearance, handling the item is enough to give a character a sense that something is extraordinary about it. Discovering a magic item’s properties isn’t automatic, however.
Because the item requires no attunement or activation, the passive quality of the +X is always on and active.
But does the player know?
This question gets a bit deeper into the problem, and the answer is that there doesn't seem to be any direction on this other than the paragraph quoted above.
It seems to suggest that using it gives the player an idea that there is something magical about it, but knowing what it is isn't automatic and generally falls under the Identification rules (Short rest, Identify spell, or both if using the Variant on page 136.)
How you want to roleplay that discovery at your table will be up to you, but the DMG seems to imply that immediate knowledge is not granted without first identifying it (but they could 'tell' that there is an improvement while using it.)
This could mean the player doesn't know and the DM adds the modifiers, or you could let the player know ahead of time (but that's also kind of a free identification.)
Yeah, I agree with the others. For me the best way is letting them experiment with the weapon, trying to focus on it and commanding, for example, to catch fire. On the other hand, you could give some hint to the player with an Int check, describe the unnatural sharpness of the blade, or describe an arcane gloom/aura emanating from it.
Then a player can figure out how to handle the weapon like his character would: a brute barbarian may use it regardless of hidden proprieties, a cunning rogue may try to sell it, lying about the magical power of the weapon and exaggerating it...or they could just cast identify.
Just let them play with the info they have.