Casting Demiplane allows the caster (warlock or wizard… or sorcerer using Wish… &/or sneaky bard) to have:
A Medium-only door (‘Must be this size or smaller to ride – NO OGRES ALLOWED’) that vanishes when spell ends / after one hour.
Creates a permanent (30′ x 30′) room of stone &/or wood walls.
This spell has amazing uses yet serious limits which others have questioned:
Question: “How much one can customize the interior of this spell?”
Answer: “If / as / so much as the DM says / so!”
Question: “What the nature of this demiplane for location & tracking purposes?”
Answer: “They must describe it a bit &/or use a ‘tuning fork‘!”
Information on Demiplane is amazingly light, especially for a game system that prides itself on doing the absolute minimum possible for a spell’s casting. This spell is neither useful for combat mechanics nor very clear for role-play purposes.
My concerns with this spell description & use:
Does this shadowy connection door between Demiplanes vanish within the hour? The spell description does not specify, so this may be a way to build up a massive interconnected demi-mansion. Can this door stay ‘open’? Can it be a ‘window’ instead? Can the connection between planes let larger creatures through if you double it up multiple shadow doors in one spot?
What happens if one adds plane-space expanding magic to this Demiplane? Say you cast a Mordenkainen’s Magnificent Mansion in there – does the whole plane become a bit bigger? Or does this simply not fit? Or does this spell trigger a collapse-explosion similar to sticking a Bag of Holding into a Portable Hole? Can one change the time-passage of the original Demiplane so as to make these space-holder-builder spells permanent?
The 5e spell specifies the new parts must be cast OUTSIDE of the original. Is there ANY way to add or expand to the space-size of this 30′ x 30′ plane? Possibly the repeated use of the (non-exhausting) Wish spell? ‘You need not meet any of the requirements’… does that allow one to add bonus space to this walk in closet? Or even make this silly vanishing smoke-door a bit more durable.. or big enough to fit a donkey through?
Base question: Demiplane feels small / simple / specific / stuck. How does an Arch-mage add bits / expand / give it more space for activities?
Answers may include: any of the guides (Volo’s Mordenkainen’s, etc.), tweets from Lord Crawford, Sage Advice, any 5e Unearthed Arcana… or whatever one can find that is RAW, quasi-legal or even slightly more tested than outright home-brew.