“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.”

Well, Planeshift can use a teleportation circle as a way of specifying a precise destination point, so maybe that helps?

It always strikes me as strange that, as written, the coaster of this spell cannot simply end it whenever they like (maybe requiring an action, or a bonus action). Of course, DM’s fiat means that this can be handwaved table-by-table, but RAW the spell has to run the full hour, whether that’s convenient to the caster or not. 

Anyone else find this peculiar? Is there an errata about it anywhere? (I’ve not found one).

Quote from Matt77stock >>

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

Because the demiplane you create with this spell is permanent, and Demiplane is much more versatile than Mansion. And you can build up an entire network of demiplanes, whereas that's pretty much impossible with Mansion since it eventually disappears.

wtf happened to this spell, it used to be good sized

What if you were to put a lichs simulacrums in a demiplane?

Well, what if you cast teleportation circle inside the demiplane? If you maintain the sigil sequence, then theoretically you could have a permanent extradimensional storage room that you can access at any time.

Alternatively, what if you cast magnificent mansion inside the demiplane?

I'm not sure it's clear if another wizard can open it while you have it open. He might have to wait until your door shuts down. I guess it's a dm call. If it can be open in two places. How does it know where to send a creature when it exists. I woul think the entry/exit would always be in the same spot within the demiplane.

I'm not sure that it's clear that two casters can access it at once. He might have to wait for you door to shut down before he can open his. I guess it's a DM call. If two can connect to it at once, how does it know which place to send a creature that is exiting? I presume the entry/exit would always be in the same spot in the plane. The spell says nothing about being able to choose your entry point into the plane.

One couldn't teleport to a teleportation circle within a Demiplane from the Prime Material.  Even when the door is open, it's still another plane, and the spell doesn't cross between planes.

Correct. They are misusing the spell.

Quote from LiloTheDM >>

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.

Quote from Goldenflame >>

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?

Quote from Matt77stock >>

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

Quote from Goldenflame >>

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?

 Correct. They are misusing the spell.

Quote from Matt77stock >>

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

 Not at all. The Demiplane exists permanently, and you can open it from anywhere with another casting. Magnificent Mansion creates a new temporary plane every time. You can't store stiff in it and retrieve it later.

I kinda think that would be an good idea to have like an meeting of important persons or villains

is there anyway of getting a sorcerer a demi plane or pocket dimension to merge with finger of death and create a zombie army, purely because i prefer sorcerers to warlocks and wizards

 and also you cant physically get to spell level 8 as a warlock, they max it out at 5

Duration 1 Hour? So what happens then? Door dissapears but Demiplane remains? So permanent demiplanes just missing doors?

Quote from HollowB55 >>

Duration 1 Hour? So what happens then? Door dissapears but Demiplane remains? So permanent demiplanes just missing doors?

The second paragraph of the spell description explains that you can reconnect to a previously created demiplane by casting the spell again.

Load it with explosives and trap the final boss there.

Original post: and also you cant physically get to spell level 8 as a warlock, they max it out at 5 

Mystic Arcanum.

Portable major image hedge maze for illusion wizards. Assemble on sight.

Quote from RodTheBard >>

Yes, Planar Shift or Gate are the most straightforward, although Banishment should work too.

 So the spell says that 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.

so you can cast demiplane to get out of your own demiplane