Make unit invisible

[RCS12]

Ok guys, how can I make a unit invisible if he enters a region? Example I named that region "Bush00" with shrubs over the region. And if a unit leaves the bush, he will break the invisibility. Can somebody do that for me?

 

[Dawn]

Add then remove permanent invisibility or make a dummy unit cast invisibility (sorceress spell) to the unit then just remove the buff if the unit leaves the region.