Half Incorporeal? Help!

[CombatWombat51]

I'm interested in making a creature that would be, well, half incorporeal, I guess. See, I want the creature to get the benefits of being invulnerable to normal stuff, fly, ignore armor, vulnerable to force, etc, but I don't want them to be able to move through walls, and especially not move through the ground.

My problem is that I can't think of a logical reason why they'd go through normal armor, etc, but not be able to move through walls or the ground. Any ideas?

 

[The Spectrum Rider]

Well, there's thickness: "the creature can pass through any solid object less than 2 inches thick." That rules out most wallks in a medieval setting, but not armor.

Or you can specify materials: steel yes, stone and wood no.

Or you can use the D&D concept of attended vs. unattended items: "The creature can pass through any object worn or carried by some character, but not through unattended items, including such things as pillars, walls, etc."

That's what comes to my mind.

The Spectrum Rider

 

[rkanodia]

The creature's body is somewhat insubstantial, like a fog, rendering it immune to non-magic attacks. At will (and even by reflex) the creature can make up to 50% of its body completely insubstantial. Thus it can use an arm to penetrate your armor, or let an arrow pass harmlessly through its torso, but it cannot walk through walls.

 

[Ferret]

How about intangible(untouchable) rather then half-incorporeal(no body).