How does Phasing interact with the Z-axis?

Question

The glossary entry for Phasing is rather sparse, and doesn't seem to address any specific issues that could arise while phasing.

Phasing: While a creature is phasing, it ignores difficult terrain, and it can enter squares containing blocking terrain, obstacles, and enemy creatures.

Presumably, floors and ceilings are blocking terrain, just like walls, so it follows that a creature with phasing would ignore those squares. However, if a creature without flying starts phasing, it would seem that the creature is no longer standing on solid ground and would begin to fall as normal, ie: towards the center of the world.

Therefore, it seems that phasing is more of a "selective" phasing, that it can ignore, rather than must ignore. Is this the case, or must phasing creatures ignore all terrain?

How does this interact with actually wanting to move through 3D space, though? Can a creature move diagonally up through a wall, selectively treating parts of the intervening wall space as stairs or a ladder?

Can a creature selectively phase part of it's body? Can it stick its head through the floor to see what's below, but maintain its purchase on the floor with its hands and knees?

Answer 1

There are lots of ways to adjudicate this. As always, it helps to have a rationale so you can sort out the effects of unplanned events quickly and consistently. We've struggled with this in a number of different systems for years, but I'll give you the two easiest mechanisms to GM that we've used.

1. If you treat phasing as a form of limited dimensional travel to a parallel, featureless world; same elevations, but no terrain effects, plants, structures, etc. This is the closest we ever came to a rationale that's consistent with the classic TV tropes. This one is the most restricting, since you can't phase in and out of the earth under this scheme. On the plus side, you don't get sucked to the planet's core by gravity, either.

2. You can treat phasing as a state during which you can move through any solid in any direction at your base movement rates (run, swim, fly, whatever) but cannot cause any in-world effects except for limited interaction with visible light (just enough to be seen) and atmospheric vibrations (just enough to be heard). This gives you many of your classic moves (sink into the earth then rise back up, "swim" upward inside of walls, survive falls from a great height by phasing into the ground and slowing gradually once submerged rather than pancaking, etc.) without making you too powerful.

Note that neither of these allows flying or "air walking". They were designed that way to keep intangibility and flight abilities distinct. Nor do they permit partial phasing, which is a real can-of-worms. Clever players can use that one for all kinds of weird effects that can be hard on your game balance, not to mentionyour sanity.