Brass knuckles?

[MThurston]

Improvised weapon? Is it a weapon for powers usage?

[nickl_2000]

Well it doesn't technically exist in 5e. So, you would need to homebrew something or fluff it from a club.

1d4 Bludgeoning Damage, 3lbs weight, Simple Weapon

Given that it's a refluff or homebrew it would be a weapon for power usage.

[th3g0dc0mp13x]

I wouldn't say it is an improvised weapon. Simple, 1-handed, light weapon. Basically a re-fluffed club.

Brass Knuckles
Simple Weapon, Melee Weapon
1sp, .5 lbs
1d4 Blugeoning (light)

For my monk, my DM approved of me using a Cestus on top of my hands as part of my unarmed attacks.

[stoutstien]

I'd add advantage against attempts to disarm and allow spikes to change it to Pierce damage

[Asmotherion]

Mechanically, refluff a Dagger to deal Bludgeoning Damage. You won't be Breaking anything.

Even if it was an "improvised weapon" it falls into the category of "improvised weapons who look similar to actual weapons, such as a Broomstik looking Similar to a Quarterstuff. In this case, you would require proficiency with Unarmed Strike, in witch ALL characters are proficient with.

Now, Mechanics and RAW aside, it's up to your DM to actually allow you to have them in his setting. In a Lore Corect Medieval Setting (And don't bring magic into it please), Brass Knuckles may not be appropriate. On the other Hand your DM may have no problem in allowing them, or the setting may be more permiting for them, so just go for it.

[Man_Over_Game]

To make it work with Monks, I'd have it not be a weapon, but be a gauntlet that said "when you hit something with an unarmed attack from your hands, add 1d4 bludgeoning damage to that attack". That way, it works with monks, doesn't get restricted from a Kensei option (for being a weapon with the special trait), and it's super simple.

[Lunali]

To make it work with Monks, I'd have it not be a weapon, but be a gauntlet that said "when you hit something with an unarmed attack from your hands, add 1d4 bludgeoning damage to that attack". That way, it works with monks, doesn't get restricted from a Kensei option (for being a weapon with the special trait), and it's super simple.

Adding to the damage has potential balance issues, a better solution would be to have it replace the normal unarmed damage with 1d4 and treat it as a monk weapon. Treating it as a kensei weapon runs into problems as you get benefits for wielding a kensei weapon and still using your unarmed strikes. For monks to benefit from the weapon, I would allow variations to change the damage type and enchantments to do bonus damage.

[Zhorn]

The previously suggested 1d4 weapon is as good a suggestion as any.
The Anchorite of Talos in DoIP have a similar weapon in their statblock, Clawed Gauntlet 1d4 slashing damage.

[truemane]

Metamagic Mod: are brass knuckles effective against the UNTHREAD?