We actually don't even know whether or not he had one. Believe it or not, Vecna has never been seen in lich form, ever. His first appearance in statted form was not until the later part of 2nd Edition, where he was already a demigod. He's also noted to have become a lich "by unknown means," though the general obscurity and handwaviness of the lichdom rituals means I can't attach strong significance to that line.
If he did have one, it's likely that it would have some meaning, though certainly not the normal significance of a phylactery. Of course, as god of secrets, tracking it down would be an intensely challenging task.
I would imagine just how vast depends where you're aiming to go, but I would expect it's not a short jaunt to use it.
As someone noted above, that's rather like "deep sea" vs. "water surface." Deep Shadow is infinite and well beyond "Border" Shadow in scope.Is Deep Shadow comparably smaller than regular Shadow or is that a misnomer?
Pfft, no.If one were to chart a course through Deep Shadow, would that chart be useful later on? Is Deep Shadow theoretically mappable?
I assume "specifically" does not mean "exclusively" or this will be an extremely short answer. You could expect most of the more dangerous natives to inhabit that part of the plane - nightshades, shadow giants, etc.Do we know of any particularly powerful entities, gods or outsiders or whatnot, that reside in Deep Shadow specifically?
The dark deity Erebus may reside within Deep Shadow, though given that he has designs on the Material Plane and his agents there it's more likely that he operates in "Border" Shadow.
Besides the possibility that the beachhead in question simply gets swallowed up by Deep Shadow, there's the issue of the attentions it might draw from things within that place where the shadows are not cast. Also, anything that routinely admits light into Shadow... well. At the very least, it'll be a beacon for things to begin lurking around on the Shadow side. Shadow in general is not a particularly stable place to attempt any kind of settlement or foothold - s'why they're so rare.What would the harm be in using magic and permanent Gates to swiftly build a beachhead, or even a small crude city, in Deep Shadow as a base of operation?
Not sure what "Astral" represents in this context.I assume it is a 'VERY BAD IDEA' but how bad? How bad on a scale of Astral to Far Realm?
I can't imagine gender is of tremendous concern to most balors. I also don't see why there can't be female balors. I don't know that either "gender" would possess discernible sex characteristics - and of course given that they're chaotic in nature, odds are decent that they could alter those as desired. Do you consider vrocks single-gendered? Hezrou? Nothing stipulates that they are.
As for mariliths, as far as I know they are all female, though the demon Verin can take the form of a "male marilith." Perhaps their appearance and nature is an echo of the dark goddess Kali, who resides deep within the Abyss. Perhaps they are actually genderless or consider themselves to be of other genders and merely happen to display physical characteristics associated with female humanoids.
Nope. Could always do something like an amnizu cleric, though that's still going to be a fairly high-CR thing, since no standard devil has native cleric casting. Alternately, just make it a unique devil that happens to possess such a capacity - perhaps a particularly specialist kochrachon or excruciarch who possesses the ability to bring his victims back against their will provided they had his venom in their bodies at time of death (presumably this would be a willing participant, so no venom needed).
Valuable secret shared or timely warning provided, or she might have somehow acquired its true name or a dangerous fragment of its soul, or she may simply hold a position in Hell's hierarchy that provides her blackmail leverage.Currently it's between a Pit Fiend (but why would a Pit Fiend owe a lowly Erinyes so much as to be willing to use his 1/year wish?)