A Demesne is a place of power and one of the three essential things for a practitioner to have. In the event that a practitioner becomes to attached to their demesne they may become a part of it and die when it fades away, resulting in a ghost, and/or a location saturated with power. Otherwise they fade into Limbo.
To gain a place of power the practitioner must stake their claim and then defeat all those who would challenge them over it. By defeating all those who challenge them, the domain is the practitioners to do with as they please, but the bigger the location to more challengers. During the ritual they cannot leave and it may take hours to days to complete it.
During the challenge to claim a place of power the uncooperative Spirits and Others are ejected, leaving only cooperative ones, but if there are no challengers then there is no way to import power and makes it useless.
Inside of a demesne the practitioner is 'A step below God' and they have ultimate power as the domain is a reflection and an extension of the self, a safe place that they can do nearly anything in. That being said Familiars have a claim to the demesne as they are a part of the practitioner. If the differences cannot be reconciled then the power that the familiar proves and be concentrated in a dedicated space.
It becomes possible to manipulate time while inside, but once out of it reality will correct the difference. As one can determine the rules within their realm, they can use the place as a form of esoteric moneychanger, changing one kind of power for another, and a source of good karma. Some find that they can draw on their continual connection to their place of power to access it from remote locations, using something like a certain key in any lock, an outline in chalk, or using the blood of an enemy.
When forming contacts with Others, the demesne becomes a meeting place and even a home to some beings, who give tribute in turn, by way of power, gifts, or services.
↑ What if you make a vehicle a demesne?
Demesnes can't generally move. There are other ways to go about it, which wouldn't be termed demesnes, but generally at a steep cost.
It's like how some eastern practitioners have their own systems & standards, where they have familiars and implements wrapped up in the same system (generally with multiples or sets). You might go this route, but there's caveats - at least 2/3 of these are liable to crop up, but very likely all 3, depending on measures taken & how much the practitioner puts into the ritual:
You might go for a mobile demesnes but you're basically wrapping it up in familiar or implement. So you have a motorcycle as your cradle, but it's also your familiar, or you have a van as your cradle but it's also your implement. It's not ~actually~ an implement though, but it's taking up that space in terms of your self definition, because it's just this clumsy thing you're doing, and it's not an equal proportion of power. This means you're basically, instead of having a familiar at 100%, an implement at 100%, a demesnes at 100%, you're going 50/50, 100.
It's exaggerated further in that it's nontraditional here and so there's less respect and focus afforded to it. So it takes a hit and winds up being more like 33/33, 100 in terms of general puissance/power level. The end result is you've almost halved your overall power level compared to what you could achieve otherwise.
Remember Blake talking about how the tool represents how people approach the world, the familiar represents the relationships they form, and the demesnes represents the position they occupy in it? If you're saying "The place I hold in the world is living in a car" or "the place I hold is that of a traveler" then you're sending a lot of signals to the spirits. There's a pretty heavy quality of life cost in this. You're signaling to the spirits that hey, you're probably never going to own a house or live in an apartment for more than a few months. Now all of a sudden you're getting evicted or the landlord wants to stop renting so they can move family into your apartment, or you just can't get that mortgage approval or get a realtor to answer your calls. There's subtle effects to how people & Others see you too. To Others, the practitioner with a sailboat demesnes isn't worth dealing with because they're not going to be here next week.
Tradition matters and you can buck tradition but at a steep cost. Generally speaking, trying to get tricky with it isn't worth it - some niche cases exist and some people don't care about overall power level, they just want a talking motorcycle that makes their nomadic lifestyle more comfortable. But you're not going to have a van and be able to be a lesser god of your own domain when you crawl into the back of it.
What if it is in a high rise which gets demolished? Would fate prevent the demolishment of the building?
If you have a demesnes you generally want to actually have a claim to the territory. You don't claim a demesnes in an apartment high rise you don't own, because that leaves you without protections and generally fucks you when you get evicted and the spirits go, "Whelp, yeah, sucks, but poof, demesnes disintegrates, bye." Most rituals are just going to require ownership of the place (or for the place to be sufficiently neutral) as a matter of how they're enacted.
Outside of the mundane world, if you own the space, have claim to it, and everything else, it's generally pretty darn hard to actually demolish. It's an extension of the practitioner and it's a rare, rare case where you'll actually be able to do substantial damage to the demesnes without also defeating the practitioner who lays claim to it... and once you pick that fight you're fighting a practitioner on their home turf. Explosives aren't liable to work right, fire doesn't kindle, flooding does minimal damage, holes in walls self-repair, yadda yadda, and while you're trying to knock down a building that's resisting the effort, the practitioner gets a heads up and then suddenly appears behind you, having arrived faster than they should've.
Can you not have a demesne where a dead person had one?
Not generally. Certain families have different rules & whatnot, so they break from tradition/normal rules, but just for the sake of argument & expansion, having the little convenience comes at a cost. If you're laying claim to the space that's been in your family for a while, then, well, it goes back to the identity thing I brought up above (third bullet point). You're identifying as someone who follows the path & place set forth by family. It might make it harder to be flexible with practice, and it'd impact how Others look at you. You're less a person and more a bloodline with a power & position that's currently resting in a different body than it was 30 years ago. The demesnes itself might be more powerful but less customizable, with each person to take it on being able to make only a few changes to make their mark on it (and less so with each subsequent generation).
Can you have an underwater demesne? What about one floating in the air, with no structure?
Generally we're solid beings who occupy solid spaces, not spaces of air or water or energy. You could dress it up as one of those, which is probably going to be easier to set up and more comfortable.
As to other options, well, ~yeah~ you could. But if you've noticed the trend outlined above, very often the cost is going to outpace the benefits.
And I gotta ask - why would you even want to? You want a place that's convenient to get into. Even if you have practice that lets you swim or fly up to some door in the sky... that's basically you setting yourself up to pay a toll every time you want to go home.
Does the near-omnipotence extend to the walls? How far into the walls?
Yes. Far enough to have windows.
Can you have multiple or expand the original?
Not generally. Not given what demesnes are, what they represent about a practitioner. You could ~try~, but on top of the refrain in this post about how costs would likely outpace the benefits (even more so than trying to have an airplane demesnes) I'd say that if I were GMing it or writing the story, I'd include a very real chance that attempts to add multiple demesnes or expand the existing one just see you lose the demesnes & end up forsworn on your original statement of claim, with a corresponding, permanent loss in overall power, extending well beyond the initial 'forsworn' penalties.
It's just not what a demesnes is. - Comment by Wildbow on Reddit
↑They are, but keep in mind the power of tradition and culture, especially with the big things & the personally defining things.
You can get a face tattoo and you know, you ~could~ get virtually any job or position, you could date people just fine, meet the parents, start your own business, whatever... But it's going to be a lot harder than if you didn't have a giant rooster on one side of your face.
...That is, unless you live in a country & place where face tattoos are commonplace.
Ditto if you live out of a car & are known to be that guy who has no plans of ever getting a house, to just paint a picture demesnes-wise. - Comment by Wildbow on Reddit