Have an Idea- Need Critique

Then what I said holds true; it is not at all overpowered for a high-level party (lvls 11-15). At extreme high-levels (17-20) the portal becomes irrelevant since the party will have access to the spell gate.

All this certainly sounds viable and not overpowered theoretically. Sounds like you’re looking more for a deus ex machina and plot device than an item; building a campaign around the shifting demi-plane, exploring it, and where it can take them. So your question becomes about how you use it in the campaign (and as a fellow Who fan, I feel obliged to support it!)

I’m also a preparation-maniac, so bare with me if I go on...

The game balance issue seems to have been explained well enough above; especially if it’s rather unreliable or you set sufficient difficulty on the skill check (either way the DM has a lot of licence for interpretation).

Personally, I’d probably make the skill roll Planology instead and have the console be an enormous planar etherescope, but working in reverse: when you set the ‘scope to represent the appropriate plane, you travel there (that gives the PCs a reason/need to find the appropriate settings for an specific/obscure location – research in the Fraternity of Order libraries or possibly even scouting it out first).

Mysterious artefacts and strange and bizarre contraptions fit right in with PS, but stylistically you’re probably best avoiding any obvious references, like referring to it as a TARDIS, as it will just disturb the verisimilitude – done right, your PCs should never catch on (on the other hand, depending on the tone of your game and how much your PCs are ‘in on the gag’ you can pepper the whole thing with subtle or blatant references – long scarves, mechanical dog golems, armies of modrons screaming “EXTERMINATE!”, and the mysterious man they call ‘The Guvnor’, etc)

It’s probably best to keep the device as mysterious as possible (plenty of precedent for that), but decide on a few operating rules and some history for the PCs to discover, so they don’t think it’s just a plot device. Things like:

-How does it actually work?
-Who built it and what clues are there to its origins?
-Is its original purpose different from its current use?
-Are there any unseen dangers involved in its operation? (like ‘parasite demi-planes’ perhaps?)
-Where has it surfaced before?
-Who knows about it and is after it for themselves?
-What results would Bardic Lore/Legend Lore etc produce?
-What’s the ‘feel’ of the place? The class and faction of its creator is probably the most significant factor here.

I don’t see there being any major problems in giving it to low level characters (unless they let someone more powerful steal it). They’ll have enough problems trying to get the damn thing working right with their limited skill scores, and exploring the maze of corridors that connect the major ‘rooms’ and the various hazards/wandering creatures (like angry owlbears ).

Some other things that leap to mind:

- What spells / flavour of magic does it use: bardic, arcane, clerical, raw divine power from a captured demigod? This should affect the flavour of the place.

- Remember that any spells the key uses to summon the entrance portal will be affected by the magic-limiting Rings of the Outlands, and possibly by other effects on various planes/realms.

- How often can the controls be used? Is the number set, or does the ‘recharge time’ vary depending on where you are? (are you going to be stuck in Demogorgon’s throne room for an hour, day, week...)

- How long does it take to activate the controls?

- Does the entrance portal you ‘summon’ appear right in front of you? In the nearest bounded space? Or in place of the nearest portal? How long does it stay for with each use? Does it disappear as soon as you go in/stop waving the key around, or does it stick around like any other closed portal, until you move the device? (I like this option, as it gives PCs more to worry about)

Also, if it appears in the place of a normal portal, what happens when someone tries to activate that portal? Does it simply not work, or is there a chance that they could end up inside – is it effectively a ‘portal shifting’ device, so any portal can be made to lead inside the machine (I like this option too).

- How easy is it to break in? My only game balance worry is that the stronghold sounds pretty impregnable and I’d expect the PCs to retreat there to camp and whenever they’re in trouble. I’d assume that any portal opening or locking spells can affect the doorway.

(If they’re clever, it also means they can take armies of cohorts about with them)

- Parasite demi-planes: These sound like a fun concept, and the ability to have weird areas where odd rules apply has a thousand uses. But what’s attracting them? Are they a planned feature or a side effect? Does it attract existing demi-planes or do they ‘bud’ from the main source (some kind of reproduction?). If you go with the idea that the machine is in some way intelligent/psionic (like the TARDIS) then is it the machine itself that is luring the demi-planes in? What happens if it snares Neth, the Plane That Lives, or when their original creators turn up?

Great idea
Lots of stuff to think about
Hope I’ve helped

This is a good idea, and a perfect device for transferring low-level PCs around the place without relying on scrolls of Planeshift and whatnot. The unreliability of the machine/demiplane allows a certain degree of control for the DM and the mysterious nature of its operations prevent the PCs from completely hijacking it. I think the demiplane should only function at intermittent, unpredictable times closely aligned to the speed of plot.

The good Doctor continually found himself stranded in strange places until the local problem was resolved, then whatever malfunction the TARDIS was experiencing was quickly resolved. Being sent to Orcus' throneroom, and having to survive for 2 days until the demiplane is "repaired" would make for an awesome adventure for low-level PCs. They obviously couldn't just hack their way out!

Actually, I used a similar concept in a non-Planescape game I ran in high school, but the machinery of the transport artifact required various things to activate, akin to gate keys. Your players have to be really adventurous though, because as Armoury99 said the temptation would be stay safe inside the demiplane and never come out.

Heh. What a fun idea, especially since some of my fellow roleplayers have recently tuned in to the New Doctor Who episodes. I'll have to use The Guvner some time in a game, even if it's just a cameo.