Create an unaccessible demiplane with Genesis

[Conradine]

A solitary Wizard cast Genesis, then find a way to have the only existing portal for his demiplane destroyed while he is in ( mabye a summoned monster, a golem left behind or a timebomb-like spell ).

The demiplane is now totally unaccessible?

[Bronk]

A solitary Wizard cast Genesis, then find a way to have the only existing portal for his demiplane destroyed while he is in ( mabye a summoned monster, a golem left behind or a timebomb-like spell ).

The demiplane is now totally unaccessible?

Well, the 'wish' spell would still let anyone in. I'm sure there are a number of other methods as well.

[Clistenes]

You know, I have always found the accessibility of Demiplanes a confussing issue. We know that Gate allows you to enter a Demiplane, but only if its owner allows it.

Plane Shift should allow you to enter a Demiplane if you have the right tuning fork, but some descriptions of Demiplanes imply that, since they are of limited size, you can enter them only from points in the Prime Material (or other Planes) that are coterminous to that Demiplane (for example, if a Demiplane occupies a position in the Ethereal Plane that "overlaps" with a house in the Prime, you have to be inside that house in order to access that Demiplane with Plane Shift).

On the other hand, Outer Planes AREN'T coterminous to the Prime, and you can still access them using Plane Shift. The Astral overlaps with both the Prime and the Outer Planes, and Plane Shift uses the Astral as an interface... and the Astral plane also allows the existance of Teleport spells... So maybe you can Plane Shift from any position in the Prime to any non-protected Demiplane?

But Plane Shift, according to some interpretations, only allows access to the first layer of an Outer Plane, so the spells has limitations... If Plane Shift allows access to Lunia but not Jovar, it would make sense that it allows direct access to the Ethereal Plane but not to a given Ethereal Demiplane...

Anyways, I think the wizard trapped within the Demiplane has another option besides the Wish spell: He or she could Plane Shift to the Ethereal Plane and travel through it until reaching another plane. Or he or she could use Plane Shift twice, once to get out of the Demiplane and into the Ethereal, and another to get out of the Ethereal and into the Prime or another Plane.

[ExLibrisMortis]

The 'transport travellers' clause of wish is both a highly specific rule (one spell only) and a very broad statement (transport regardless of local conditions). There are very few counters to it by RAW (none that I know for sure), but you can always try talking to your DM. As options, I would suggest:

1) Casting genesis in a specific unconnected place, such as Sigil, the Observatorium (Manual of the Planes), or the Far Realm.
2) Block wish by using a mythal (epic magic).
3) Block wish with a planar bubble keyed to a specific place, such as Sigil.

All of these are various grades of DM fiat, but they can still be practical in an actual game.

[Flickerdart]

1) Casting genesis in a specific unconnected place, such as Sigil, the Observatorium (Manual of the Planes), or the Far Realm.

This won't work - you can only make arcane/divine demiplanes on the Ethereal, and psionic demiplanes in the Astral: "A character can only cast this spell while on the Ethereal Plane. (http://www.d20srd.org/srd/epic/spells/genesis.htm) "

[Lord Vukodlak]

Alright lets look at the spell Genesis,

Said demi-plane is coterminous with the Ethereal plane. But that doesn't actually mean much as the Ethereal Plane touches everywhere.

The important factor is they are demi-planes
Meaning you can reach it from the Ethereal plane, IF you are at the point where the two touch. The Old Manual of the Planes is pretty clear as to demi-plane access. You need to be near the point where the two planes touch in order to plane shift but that's a demi-plane restriction not a general one.
So the wizard inside the demi-plane could planeshift out without a problem, but would need to return to the point his pocket plane overlaps with the ethereal to return.

I know Manual of the planes is 3.0... but I think the rules for how planes function hasn't changed

The spell genesis doesn't include creating a portal. You need to cast spells to access it to begin with or create your own portal. To make it inaccessible you'd need to ward the plane against planar travel. There are plenty of methods to do that. Also the focus for planeshift has no cost so a component pouch lets you ignore it.

Personally I ruled the psionic wording of Genesis to be the only one, no matter who casts it. Simply because it leaves fewer questions.

[MaxiDuRaritry]

See, what you do is, you craft the demiplane such that it's got minuscule tunnels throughout, similar to an anthill, and made from riverine (to prevent incorporeal shenanigans), and if you want to enter the plane safely, you must be ant-sized, because all the tunnels and rooms are Fine-sized. Make sure the demiplane's got the timeless trait in regards to magic so you don't have to renew your polymorph spell. (Make sure you're always invisible or otherwise out of sight before you enter your demiplane, to prevent others from learning the secret.) Anyone too large to fit will appear in the only place in the plane large enough to hold it -- which just happens to be a Gargantuan-sized hole filled with quintessence. If you're not Fine-sized, that's the only place you can appear, so any unwanted guests will be pulled out of the timestream, with no way to prevent it. How do you get quintessence as an arcane caster, you ask? Why, from a secondary demiplane you created that is linked to the first one, which is made from quintessence. Any unwanted visitors, you just push in the portal to the second plane to get rid of them. Make sure to wear spellblade rings of wish, miracle, and reality revision, to ensure you can't be pulled out of the plane without your permission.

[edit] Make sure you have the regular quintessence pool overlapped with an incorporeal quintessence pool, so nobody can escape their fate by going incorporeal and passing right through the quintessence.

[ExLibrisMortis]

This won't work - you can only make arcane/divine demiplanes on the Ethereal, and psionic demiplanes in the Astral: "A character can only cast this spell while on the Ethereal Plane. (http://www.d20srd.org/srd/epic/spells/genesis.htm) "
I know that, I'm just suggesting a specific strategy that a player might suggest to their DM, as fluffy reason for disallowing entry to their demiplane, up to and including wish-based travel. The other two things I mentioned don't work either, by RAW (at least, I'd guess the presence of a mythal or planar bubble-engendered dead magic zone counts as 'local conditions').

[Gildedragon]

Regardless of what one does: A portal from sigil; the world serpent inn; the infinite staircase; or other infinite portal spaces is bound to open into the plane.

[ILM]

Okay, here's what I had cobbled together from various sources.

The Demiplane section of Manual of the Planes (3.0 source, but I assume the fluff is still valid) states that "Demiplanes are almost always coterminous planes that are moored to another, major plane at a single location [...] they can only be accessed from a specific spot on the larger plane." It also states: "Characters cannot use plane shift or similar spells to travel to or from a demiplane, except at the specific locations where the demiplane is close to its coterminous plane. [...] some demiplanes don't even have a single portal - such places are often used as prisons for powerful creatures."

The creation table on page 155 has an entry for access, which lets you roll or choose between standard portal, keyed portal (requires specific conditions to open), two portals (one in and one out), and two independently keyed portals.

Finally, the Gate spell reads: "Deities and other beings who rule a planar realm can prevent a gate from opening in their presence or personal demesnes if they so desire." I think it's reasonable to assume that the creator of a demiplane is its ruler too, so you can just say no to Gate.

So, Gate is out, and plane shift and the like are limited to the demiplane's portal, which you can restrict access to when creating your demiplane. I assume - the wording is a bit unclear - that what this means that that a creature can either walk through the standard portal/ use the magic key for the keyed portal, OR force its way through by plane shifting in the exact right spot. This is fairly easy to shut down with a well-placed dimensional lock in your demiplane, which you'll renew every few days. Manual of the Planes states explicitly that a demiplane's portal remains functional even if the demiplane has the dead magic trait, indicating that the portal itself remains active regardless of local magical conditions, and therefore that it will not be impaired by the Dimensional Lock.

Unfortunately, as far as I can tell a Wish will still get you in anywhere, at any time. Them's the breaks!

edit: too slooooow!