I hope I'm posting this in the right place. 

I have an idea. I'm sure its considered to be a bit of a homebrew magic item but I'm not entirely sure because I couldn't find any information on items like this. Maybe because I'm not entirely sure what to call it. 

Is there a way to have an instrument that changes into a weapon. The one I want is a flute that changes into a sword. 

If this is already a type of item or a spell that is achievable then I'd love to know. If not is there any DM's out there who would allow this kind of thing as just a magic item, or any reasons why not, or what are some rules you night put on this time?

The veteran's cane is a cane that comes with a one time magical conversion to a sword.   Closest I know of.

I'm curious about this too, but for a very specific item.  I want the Rod of Lordly Might to be a flute instead of a rod.  I like the utility of the RoLM which fits a Bard's support role well.  I suppose a Lute/Battle axe combo weapon would work too.

Regardign the initial post: You could use a Sun Blade, its a Rare item that is supposed to look like the hilt of a sword that produces a radiant blade on command but there is no reason you couldn't flavour the hilt of the weapon to be a flute.

I think the problem with flutes becoming weapons is that a flute must be hollow to produce sound.  Of course, this is D&D so I suppose a sword could just materialize.  You just wouldn't want it to happen while you were playing :D

I wanted a flute to use for my monk that could be used as a club and a blow dart gun. But I wasn't sure if this would be allowed. 

I do not know about a club but......

I had a gnome make me two recorders(remember those) that the mouth piece came off and the two tubes pushed together to form a 2.5 foot blow gun. They are not long enough for a good blow gun alone.
The holes were fake and didn't go all the way through. And to imitate music I would use minor illusion or prestidigitation to make the sound if I wanted to look like I was playing it.

You could also use it to breath under water. (two feet under the top) Or if you could have a u-bend shaped pipe to attach to it and make it a snorkel. If your DM allows it.

Its not magical but would serve a use.

Just remember that a blowgun is only as good as the poison your using.  Essence of Ether, Oil of Taggit, Torpor and Carrion Crawler Mucus are my favorite. Not all usable by blowgun. But use Mage Hand for the others if possible.

I would not allow a non-magical flute/blowgun/club combo.  That's just not something that would actually work- the properties of each are too different to make a functional hybrid device.

I would allow a flute/blowgun combo, particularly in that it would have a secondary barrel built into it, making the flute slightly bulkier but also letting it pass scrutiny as a flute. To shoot the dart, cover a certain chord and blow, and the air would pass down a second pipe and shoot a tiny dart. Darts don't have to be big, after all!

I would not allow a non-magical flute/club, as flutes tend to be light and thin, which a club is not. A Digeridoo Quarterstaff could have legs though.

revisiting this topic I had a little looksy on a certain video site and found flute/blowguns were actually a thing. Apparently Ninja employed them and they seem to be in two versions; 1) pulled apart just below the holes in the flute, the dart would be loaded and then shot and then the flute pushed back together and 2) the flute had a an internal "barrel" that could be rotated from the mouth end of the flute and this would cover the holes allowing the dart to be loaded and shot. Either way it was damn clever stuff from feudal era (possibly earlier) Japan.

EDIT: for the club side of things I suppose you always have the good old option of Tavern Brawler to use it as an improvised weapon, although possibly with onyl d4 damage instead of a clubs normal.

EDIT TO THE EDIT:  I also found something about a "Silent Flute Form" that was developed by Chinese monks where they used flutes made of wood, bone or metal as weapons whilst travelling to fend of bandits so a club flute might not be terribly far fetched.

A club's damage is 1d4.

Quote from Rob76 >>

revisiting this topic I had a little looksy on a certain video site and found flute/blowguns were actually a thing. Apparently Ninja employed them and they seem to be in two versions; 1) pulled apart just below the holes in the flute, the dart would be loaded and then shot and then the flute pushed back together and 2) the flute had a an internal "barrel" that could be rotated from the mouth end of the flute and this would cover the holes allowing the dart to be loaded and shot. Either way it was damn clever stuff from feudal era (possibly earlier) Japan.

EDIT: for the club side of things I suppose you always have the good old option of Tavern Brawler to use it as an improvised weapon, although possibly with onyl d4 damage instead of a clubs normal.

EDIT TO THE EDIT:  I also found something about a "Silent Flute Form" that was developed by Chinese monks where they used flutes made of wood, bone or metal as weapons whilst travelling to fend of bandits so a club flute might not be terribly far fetched.

 

 They also used paper to cover the holes. Either inside or outside the flute.

So, it sounds like it isn’t against the rules, it will be down to the dm. If I find one.

The new D&D movie statblocks include a "reinforced lute" for the bard character, Edgin Darvis, that doubles as a weapon.

Comments from the peanut gallery - again
* there is a homebrew magic item already on here that is a guitar that converts to a battleaxe called Heavy Metal
* while the oldest flutes are single wooden items with holes along their lengths, modern (transverse) flutes are typically metallic and consist of 3 fitted sections so it’s quite possible for them to be used as both blowguns and clubs. While modern flutes only date from the 1830-1850 period there is no reason that you couldn’t have a transverse or vertical flute made of metal at earlier periods and have it made of multiple fitted pieces as modern flutes are - especially for a traveling bard. The walls of modern flutes (and by extension fantasy metal flutes) are thick enough to act as a club under duress. I’m sure I wouldn’t want to use it as my primary weapon unless it was magical and could then be assumed to have the strength to take on other magical weapons.
*Which leads to an interesting thought - as a magical item that is not only a musical instrument but also a missile and melee weapon it should be able to act as a spell focus for a bard, grant it’s magical bonus to the spell save DC/Attack bonus and to the attack and damage bonus as both a missile and melee weapon - quite a powerful item, especially at +1.