Most of us know about the null terrain that units can walk on. However, the units can then travel over any terrain (I think) until it reaches normal walkable terrain. Is there any way to block the unit from going anywhere and keeping it on the black terrain?
With triggers

Can you be more specific? And does this involve a lot of locations?
Make alll the terrain surrounding it high ground, and all of the places you can get off medium/low. Have a location that only detects high ground, make the triggers, and you're set. I have never tried this, so I am not sure what it detects the null terrain as, so if this did not work you would have to make units or locations surrounding all areas of the null terrain.
One problem though, what is Black terrain? High ground, medium ground, or low ground?
You'll have to test it out to figure that out. Like I said, if it is all 3 or something weird, you'll have to substitute it with individual locations or surrounding the entire area with buildings/units the unit can't walk through.
I think I heard someone mention that null terrain has the properties of the row it's in...