I was thinking that because the Undervault is such a large area, and is a separate continent from the rest of Prime, it should have its own room in the cubix fulcruxes for people to exit from. Maybe not as many exits as the Prime Plane itself (since it's smaller, I believe), but some to make it slightly easier to navigate. Or maybe just one exit period to make it easier to enter the place if one has a cubix. You could have the room below the Prime Fulcrux because it's, well, under those exits.

Reasons against: Cubixes don't work underground. If you use your cubix to get there, you're stuck unless you have some alternate method out. It'd be interesting to conjunctio someone to follow you, lead them into the Underground, lose them, and walk out... and so on and so forth.

Reasons for: There is a cubix exit to the Catacombs of Corpus Clay, which are indoors. If you enter there, you can't cubix out again without running to the Mountains of Madness or transversing to Ethereal. This could be used as a precedent to nullify one of the reasons against.

To be honest, when I saw the topic title, I thought you were going to be suggesting a whole 'nother cubix that only worked indoors, and went to Undervault locations (works on Prime or Undervault only).

Would another indoor cubix like that be more logical/balanced?

I'm not sure-- I was still speculating on pros/cons of it while I read your post, and hadn't come to a conclusion yet. It's got possibilities-- and it also means "more credits spent," which is always a plus for a business... heh.

Making a cubix work indoors, however, is a no-no. Making off-plane insta-transport work indoors in general, I'd also have to say is a no. That's a downside to the artifacts, which they do need imo.

Maybe make it something like a Prism-- 800 credits, only works in the Undervault, and lets you get to other Undervault locations. You'd still have to walk on Prime to an Undervault entrance, or use a cubix/prism to get to one, or one of the many other transport skills-- but it'd let you move around much more easily down there (which, given its size, I do believe is necessary).

The Aetolian equivalent of a cubix does indeed come in two varieties, one for outdoor/aboveground, one for indoor/belowground.

Well that's only because of the way vampires work, I believe.

Paradox puzzle pwnz.

I only meant that it's a practicable solution in terms of coding and building at the very least. Whether it would be too unbalancing (or not unbalancing enough, for that matter) is another question I didn't try to answer.