[Sombre]

You mean item modding, right?

You can currently mod everything about the armour listed in the equipment part of a unit - def, enc, protection, head/body, resource cost.

But item modding is limited.

If you want a unit that has armour that makes it ethereal, just make the unit ethereal, call the new armour "Shadow plate" or whatever and explain it in the description. If you want something 'new' you build using item forging, all you can do is use the limited mod commands to change an existing piece of item armour.