Locating and accessing a demiplane

[ErebusVonMori]

What are the ways to locate and access either a deity's or high level caster's personal demiplane?

[MisterKaws]

None, considering both of these have already cast anti-detection spells up the wazoo.

A Greater God of Magic or Secrets might be able to do it, but I doubt they'd bother helping you unless you provide suitable payment.

[Venger]

Depending on whether your target's left any ways to access his demiplane on the prime material, a silver key might be able to help you out when he's not busy eating peoples' cucumber sandwiches in mordenkainen's magnificent mansions.

[Kelb_Panthera]

Discern location seeking the coterminal point between the demiplane and the ethereal. Then precipitate complete breach (PlH). If the DM's not a d-bag, the breached plane should have a pretty good chance of being the target demiplane.

A member of the silver keys, a planar champion, or someone wielding a weapon of passage re-worked for the normal cosmology might be able to do it.

Also wish, obviously.

[ErebusVonMori]

Ok, and what protections can be put in place to stop this? And how can those be got around? Also what if you were trying to do this to a god of magic's demiplane?

[MisterKaws]

Ok, and what protections can be put in place to stop this? And how can those be got around? Also what if you were trying to do this to a god of magic's demiplane?

Just destroying the default portal makes it pretty damn hard to breach. I think you could also encase the coterminal point in lead(pretty much make a big lead ball around it) to prevent divinations. Puting the ball inside an antimagic field/Energy Transformation Field(Disjunction) would make it impervious to most magic. After that, just load it up on all nondetection/mindblank variants you can find.

On the God of Magic thing: give it up. Gods of Magic are rarely any less than Greater Deities, since magic is such a major domain. As such, they can detect any spell you can try to cast weeks before you even think about casting it, giving them all the time they need to set up a Contingent Disjunction - concealed, of course, maybe by using Invisible Spell - exactly at the point you're going to cast anything.

[Venger]

Discern location seeking the coterminal point between the demiplane and the ethereal. Then precipitate complete breach (PlH). If the DM's not a d-bag, the breached plane should have a pretty good chance of being the target demiplane.

A member of the silver keys, a planar champion, or someone wielding a weapon of passage re-worked for the normal cosmology might be able to do it.

Also wish, obviously.

I think a gatecrasher might be able to get in there too.

[sleepyphoenixx]

That depends a lot on the specific demiplane.
What planes, if any, is it coterminous to, and where (and when)? All demiplanes are generally only accessible at specific places, and some of them aren't coterminous to the Astral, Ethereal or Shadow plane at all (any one or even all three), or are only accessible at specific times. All of those can make travelling there harder.

Are there any stable portals? Are there any wards against teleportation or divinations? Is there a mythal, a Weirdstone or something like that? All of those can make travel there harder.

What traits does it have? A dead magic or impeded magic plane can be accessed, but you won't be leaving unless it has a permanent portal or you're an Initiate of Mystra. Not a situation you want to go into blindly.

There's also special abilities to consider - a Singer of Concordance 10 for example is pretty much unassailable on his private demiplane. Even if you somehow got around their class abilities there's deific intervention by Io for anyone committing violence on one, so you'll be booted out no matter what, if you can enter at all.

As for the private planes of deities, the simple answer is you don't. Especially deities of magic, because their portfolio sense will warn them long before you try and even if you do get in you can't really win there against the owner.