What does the box do?

[firelistener]

I gave one of my players a small wooden cube from a magic item salesman. They all failed horribly on any kind of checks to discern anything about it, but all I have planned so far is that it's cursed and I said, "you suddenly have an overwhelming sense that it would be a bad idea to hand this to someone else and somehow it finds its way back into your pocket" when he tried to hand it to another player character.

With that in mind, any ideas what I should have it turn out to be?

[LibraryOgre]

The box eats magic items and turns them into an equal volume of copper pieces.

The box is an extradimensional creature's ear. It can hear everything that happens within a few feat of the box. It gives advantage on checks to listen for danger, but also feeds secret information to this horrid beast, who will be around to collect it... especially if they stick anything in its ear.

[MaxWilson]

I gave one of my players a small wooden cube from a magic item salesman. They all failed horribly on any kind of checks to discern anything about it, but all I have planned so far is that it's cursed and I said, "you suddenly have an overwhelming sense that it would be a bad idea to hand this to someone else and somehow it finds its way back into your pocket" when he tried to hand it to another player character.

With that in mind, any ideas what I should have it turn out to be?

Whenever you put your hand in the box, you somehow find yourself clutching a piece of expensive jewelry... with no idea how it got there. The jewelry invariably gets recognized shortly thereafter by someone powerful as belonging to their loved ones.

[Safety Sword]

I like Mark Halls' idea... maybe a slight twist.

It's a waste disposal cube. Anything you put in there to be destroyed but actually is transported to some extradimensional space. However something lives in that space and gets mighty annoyed at all the junk that gets dumped there. Later on you find out that there are 99 of these cubes and people all over the world have been using them. The creature in the disposal dimension gets fed up with it all and comes looking for the cubes.

Maybe the creature is made of all the junk even. A junk golem?

[Imbalance]

It's a magical coffin, bound to the last person who paid the purchase price. On the day that the character fails their final death save, it magically enlarges and becomes a true burial box to fit them and teleports them inside. The coffin is indestructible and forever sealed; the occupant is immune to any form of necromancy. An illusory shrine dedicated to their life and deeds appears to anyone who examines the coffin.

[IsaacsAlterEgo]

It's cursed with enchantment magic to make it appear sinister and ominous, and controls the holder to make them hold on to it. It doesn't actually do anything bad but occasionally just torments the holder by giving them a foreboding sense of doom, mysteriously glowing red at inopportune moments, or disappearing/reappearing at the most suspicious possible moment (appearing on an antagonists belt, disappearing back into the player's bag once they look away, somehow appearing in a chest of loot that hasn't been opened for a hundred years, etc.)

Basically, the overwhelming sense of it being a bad idea to give it away is the entire effect of the item, just maybe make it increasingly more elaborate until they deal with it and can get a kick out of it when they find out it's just a wizards idea of an incredibly elaborate practical joke.

[Keravath]

It is a cube of cowardice. A wizard was originally trying to create an item that would rescue them from combat but the logic turned out to be flawed.

Whenever the character finds themselves in a combat situation with an opponent within 5', the cube teleports the character away 40 + 1d6 * 10 (50-100') in a random direction - 1d8 for direction - 1 is North, 3 east, 5 south, 7 west plus points in between. The cube won't put the character inside something, however it does not check that there is a floor, breathable atmosphere, other enemies. It also has 1d6 charges/day after which it ceases to function until the next day.

[Corran]

Once attuned, the owner of the cube starts seeing symbols on the surfaces of the cube, that no one else can see. The symbols can be moved around across the surface of the cube (which holds its shape) in a particular way (just like if it was a Rubik's cube). It's a game. Possibly designed to pass the time while sharpening the mind, but who knows what went wrong with the enchantment used and it ended up as something that mainly distracts and is addictive. At the start of every day, the owner of the cube rolls a save (probably a wisdom save against some DC). If you fail the save, you have a hard time paying attention, as even if you are not physically engaged with the cube (but why would you, at least voluntarily), part of your mind is certainly occupied with it. You have disadvantage on ability checks for the next 24 hours. You can also easily lose track of time. If you succeed on the save, you are immune to this effect for the next 24 hours, or until you happen to engage with the cube voluntarily, at which point you re roll the save.

While attuned to the cube, you always know the direction to its current location and you have a rough idea of its distance from you (an uneasy thought that may have some potential). A remove curse can break the attunement.

[False God]

I gave one of my players a small wooden cube from a magic item salesman. They all failed horribly on any kind of checks to discern anything about it, but all I have planned so far is that it's cursed and I said, "you suddenly have an overwhelming sense that it would be a bad idea to hand this to someone else and somehow it finds its way back into your pocket" when he tried to hand it to another player character.

With that in mind, any ideas what I should have it turn out to be?

The Box is a cross between the Lament Configuration and The Moment, a sentient, infinitely complex puzzlebox that inspires obsession(magical compulsion) in the owner the more they attempt to solve it.

[Cicciograna]

It's a Bene Gesserit pain box.

[Desteplo]

I like the coffin idea. But with a ring of mind shielding twist

When you die you switch places with the creature inside. Can only happen once a full moon or only once per person.

The deceased can give short vague replies when spoken to And is still immune to resurrection (being cursed and trapped in the box) a remove curse destroys the soul of the creature inside but doesn’t destroy the box or magic behind it

Box when open shows piles or an endless supply of ash

[firelistener]

So I went kinda with the first idea, but not completely yet. Gave the player advantage on all his active perception checks and just said stufd like, "you feel really great today, like you can just hear/smell/see better for some reason" when he did those things. I also had him come back from incapacitation with 1HP after he rushed the grell they were hunting. Monk managed to stabilize him, but I let him get back up. Sorcerer saved the day with a crit Chromatic Orb.

I plan on it being some kind of eldritch entity, maybe, but they're playing in a setting with a bunch of orcs that practice ancestor worship, so I think it'll be the sealed spirit of someone really evil. Anyone got ideas for what kind of person should be in it? Probably gonna be the next BBEG after they take down the current one I have.

[Asisreo1]

It's a music box that plays The Box by Roddy Rich

[Segev]

So I went kinda with the first idea, but not completely yet. Gave the player advantage on all his active perception checks and just said stufd like, "you feel really great today, like you can just hear/smell/see better for some reason" when he did those things. I also had him come back from incapacitation with 1HP after he rushed the grell they were hunting. Monk managed to stabilize him, but I let him get back up. Sorcerer saved the day with a crit Chromatic Orb.

I plan on it being some kind of eldritch entity, maybe, but they're playing in a setting with a bunch of orcs that practice ancestor worship, so I think it'll be the sealed spirit of someone really evil. Anyone got ideas for what kind of person should be in it? Probably gonna be the next BBEG after they take down the current one I have.

Maybe it's not an ear, but a container of an eye. Gruumsh's missing one, specifically.

Being attuned to it causes him to feel an inexplicable sense of shame whenever he does something cowardly or weak. Weakness includes any form of kindness that isn't rewarding somebody for boldness in your service. And with pride whenever he defeats somebody, in anything, especially physically.

It is a puzzle-box, as well. Crafted by Vecna, after he stole Gruumsh's eye. Gruumsh has never been able to solve it, and can't stand the brainy wimps who might have a chance at it, which is why his influence has kept those who might solve it for him from having it. If it's ever solved, the last step to open it requires peering deep within a crack, and the whole thing will rip out the eye that's peering in, and plop Gruumsh's in its place. Very painful, but it's a god's eye, so it works in your socket. But now Gruumsh can come for it if he doesn't think its current home serves him better....