Infinite storage space and a terrifying place to imprison an enemy. Just don't go inside your own demiplane when it closes or you're screwed.

So would a caster fill their demiplane with a random assortment of strange knick knacks so nobody can guess the contents of the demiplane and access it? If you just put the MacGuffin in there, anyone with an 8th level spell slot can just connect to "the demiplane with the MacGuffin", but if it also includes seven pink snail shells, a T-Rex's left pinky toe and a signed copy of an obscure smut novel, the demiplane would be safe.

Various ideas.

Use Finger of Death on small villages, take a few here and there creating an undead army under your total control. Open Demiplane and send them in. Fill space to brim, they don't eat, sleep, or breathe, so all good. 

Invading a castle? Approach solo, open Demiplane on other side of gate where you can see. Command army of undead to open gate and hold defenders away. 

Or, as heroes (other NPCs) reach their prime, True Polymorph volunteers into small glass statutes. Stack them in Demiplane for time of great need. Contingency each that when a specific action or event occurs, the spell is Dispelled and they come out ready to fight. 

Going to raid a treasure vault, but limited time to loot? Demiplane on the floor, open the door, shovel treasure in. Inventory later. 

As a wizard:

Already have the teleportation circle spell be known

get to 16th level

learn this spell

use this spell, create a demiplane where oxygen is not needed, you don't age while in it, and time doesn't pass, seeing as you made the plane, you get to make the rules, so long as they're at least slightly logical

also know find familiar and maybe multiclass into bard or druid so that you can get awaken

You now have your own demiplane, a way to get in and out of it, and you have a familiar and possibly several other animals with the intelligence of a human and a language you understand

Possibly become a lich, or grow your power using various means, or maybe even become the villain of the campaign, who knows

With your infinite time you could raise armies of undead, super intelligent animals, or just have fun and do cool shit

When you're done screwing around, hop out exactly the same as when you went in but several times more powerful, as time hasn't passed

Create unbeatable contingencies, learn all the secrets of the world, and everything else you could imagine in the time of two actions, roughly

Spend effectively decades coming up with a foolproof plan to do something without aging at all, and become all-powerful in a matter of seconds in the actual world

You have now beaten DnD, assuming that the rule applies where you can make up your own dimensional rules. Even if you can't, you have a dimension all to yourself that you could make your portable home, assuming you had the right spells.

So you could use contents as a combination lock, right?

Like, a sack with an assortment of odd objects? Or even a letter or written password? Perhaps a carved password or icon?

I'm thinking, depending on how strict "knowing the contents" is being interpreted in your game, you could greatly increase the security of your personal demiplanes. And by 'how strict' I mean do you only require a rough idea, a general or basic description or an exact inventory?)

And to protect your combinations from psychic intrusion by not knowing them yourself (by basically make nuclear launch codes for yourself)!!
Follow me here ... 
-You make a set of dice which have pins for pips.
-Make a sort of shaker box with slots each die falls into when shaken and allowed to settle.
-Once all the dice fall into a slot, the box can be fully closed, and press on the die (also allowing you to 'feel' if each die has settled into a tumbler.
-Below the slots is a tray you can place two pieces of parchment in. The tray has two open sides opposite each other the other two sides closed.
-You should clip one parchment to the bottom of the tray.
-With the parchment in place and the dice shaken into random positions and rolls, press down, punching holes in the parchments.
-Allow the free parchment to fall into a slotted box or something without looking, and then unclip the other and let it slip into an envelope or something without looking.
-Seal the envelope and keep it on you; leave the other in the box in the demiplane.
-Don't unseal it until you want to return and destroy both copies once you do. Create new ones before you leave (should only take a moment).

There are other variations of course... With magic, engineering, and/or sheer creativity, there are all sorts of ways to do something like this. Now, it's not without its drawbacks - your demiplane key codes could be stolen, but if you kept it's purpose a secret then it adds a layer of security to any thought-thieves and telepaths (they can only steal the knowledge of your single use key code vs the knowledge of its contents outright.

But again, how strict? If a coded message in an envelope can prevent break-ins, then you would also likely need to know the exact contents of the room as a whole since a code requires a level of specificity that would make every object in the demiplane a kind of key code w/ your envelope as the actual password to gain entry. Otherwise, you wouldn't need to know the code if you didn't need to know the exact details of every object. 

I dunno - but either way, this spell is crazy (its difficult to determine the limits of it)

And then you gotta ask yourself, could you almost use demiplane as a poor man's Wish? In the infinite multiverse, you could come across all kinds of other demiplanes loaded with treasure and no defenses simply by using your imagination!
"I know a demiplane thats just a wooden room full of platinum coins. Literally full" without having filled it yourself, or even seeing it. You just know it must exist and therefore it does for you.

That said, the rule limits itself by not elaborating on what it can do. Look at Magnificent Mansion - very specific. Perhaps "nature" is whether it appears to be made of wood or stone, and contents are what you've added to it? There's nothing else presented in the rule it could pertain to, but it's all interpretation at that point. Maybe you can determine the physical/magical laws present there? Maybe you could simply say, "it's spooky"? 

The thing is, it doesn't address the light levels, whether you can breathe, etc - maybe that suggests its literally just an empty 30 ft cube which appears to be wood or stone. If it has a door to it still, you can breathe. If not, you can't. There's no light unless you put a light in (vs stating something like, "its nature is a Brightly Lit room where breathing isn't necessary"). Just treat it like a normal space outside the descriptions presented?

Could still probably "know" infinite rooms of fancy stuff tho. Multiverse has it all!

So, for a spell you can have at level 15, you can literally create a full on demiplane, and with enough time, can create a lavish hotel-size completely portable home with all the wealth and power of an adventurer. gotcha. This isn't to say that it should be removed, it's super fun, but man, does it break the game horribly.

Okay, so because this spell is confusing, here are the rules as I understand them, as well as a few creative uses, from the basics on up:

As an action, you cast the spell. A shadowy door is created, and so is a demiplane if you so choose. The door lasts for an hour, but the demiplane lasts indefinitely. You can also connect to a specific other demiplane if you so wish. 

It is important to note that, once this door to the demiplane disappears, there isn't a way out of it unless you have magical means to open a portal up to a different plane (also, Teleport and Teleportation circle only works for the same plane of existence you're on, so they're ineffective). The only real way out, at this point, is for either somebody to know of the plane and link up to it from the outside, or for you to have and then cast Plane Shift, attuned to your plane of choice (a way to get back to your own demiplane with a 7th level spell slot instead of 8th is to cast Plane Shift to your demiplane as well)

The space of the demiplane is easily twice the size of the average room, and could allot some serious storage opportunities. I mean, filled bookcases lining a wall, chests on another, tools of all kinds on the third, and maybe some standard life things on the last, and you've practically got a hotel room (prestidigitation and Mending for cleaning and sustaining your clothes and the like, maybe have a bathtub and use Create or Destroy Water at fairly raised level for bathing, idk man).

To my understanding, you can set the laws of your plane as well. This means that you could rule that time doesn't pass, there's always a natural slightly cool temperature, and despite there being no lights there's a day and night cycle, and it's slightly longer or shorter, etc., and this can be abused heavily. 

This spell is perfect for Liches. So long as you never give the coordinates of your plane to anybody else, and you keep things to satiate your soul hunger there to breed and be harvested, you can live indefinitely with no one knowing you even exist after the people that remember you have died out. I mean, even if they do know you exist, they have literally no way to access you.

That is all. Feel free to reply with more options or to correct me if I'm wrong, and I'll edit this appropriately. :)

Quote from Riesenfriese >>

So would a caster fill their demiplane with a random assortment of strange knick knacks so nobody can guess the contents of the demiplane and access it? If you just put the MacGuffin in there, anyone with an 8th level spell slot can just connect to "the demiplane with the MacGuffin", but if it also includes seven pink snail shells, a T-Rex's left pinky toe and a signed copy of an obscure smut novel, the demiplane would be safe.

 Is that where I left that copy of 50 shades of Grey ?

How does the flow of time work inside the demiplane? If the door is closed does time still flow??

My understanding is that it completely depends on the caster, and of course, the DM. So long as your DM allows it, I'm pretty sure that the caster can fully dictate the flow of time. Also, I'm almost certain that no matter what, having the door open only matters if you aren't fully on either side of the door. I personally just let the players decide how it happens.

What a fun and useful spell.
-Be at home wherever you go
-Short or Long rest safely
-Send enemies into a Demiplane you've created with this spell using Plane Shift by attuning a tuning fork to it (Plane Shift Material component)
-Create different demiplanes for different purposes- a storage demiplane, a prisoner demiplane, a home demiplane, a panic room/recovery room
-Cast Demiplane at the bottom of a lake, pool of lava, vat of acid. Cast it again to flood an area with that substance @HailRobonia

Gate + demiplane = player-b-gone

So what lies beyond the stone or wood walls? What happens if you dig a hole through one? Can you hollow out an entire plane for yourself like this?

"Each time you cast this spell, you can create a new demiplane, or have the shadowy door connect to a demiplane you created with a previous casting of this spell. Additionally, if you know the nature and contents of a demiplane created by a casting of this spell by another creature, you can have the shadowy door connect to its demiplane instead."

Just a reminder since I see some comments that miss this, if you created the Demiplane you can ALWAYS make a door to get back to it, even if you don't know the contents. The nature/contents are only necessary to connect to Demiplanes created by some other caster. So people messing with the contents would not lock you out if you created it. So don't leave stuff in other people's demiplanes, because they can lock you out by changing the nature or contents.

Quote from yappapodcast >>

"Each time you cast this spell, you can create a new demiplane, or have the shadowy door connect to a demiplane you created with a previous casting of this spell. Additionally, if you know the nature and contents of a demiplane created by a casting of this spell by another creature, you can have the shadowy door connect to its demiplane instead."

Just a reminder since I see some comments that miss this, if you created the Demiplane you can ALWAYS make a door to get back to it, even if you don't know the contents. The nature/contents are only necessary to connect to Demiplanes created by some other caster. So people messing with the contents would not lock you out if you created it. So don't leave stuff in other people's demiplanes, because they can lock you out by changing the nature or contents.

 Well, they can still lock you out by changing the contents whether you leave stuff there or not.

A little confused why this is an 8th level spell and mordenkainans mansion is 7th? It seems like the mansion spell is better then demiplane in almost every respect, while using a lower spell slot

I'm curious, people keep suggesting putting a Teleportation Circle in a Demiplane... but to use the Teleportation Circle spell to reach one, you have to be on the same plane. So you can never actually reach a Circle in a demiplane. Yeah?

To my understanding, there's no way to use Teleportation Circle to get out of a Demiplane. I'd just recommend taking Plane Shift at some time before or at the same time as Demiplane, it's way more useful and has far less potential to backfire catastrophically.

https://www.youtube.com/watch?v=Am0mWULgmuE