how to reduce vision?

how do u reduce the vision of a group of units in an area?

There's an item ability called Item Sight Range Bonus. See if you can get that ability to owrk with a negative bonus. If it works, then just give the ability to all the units in the area to decrease their sight range.

If not, you can give them the ability in the begining and reduce the site to match what you want it to be. Then take away or de-level the ability to reduce the sight.

Negative works as long as the unit doesn't have an ability that detects invisible units though