I think the [ljass]Ghost[/ljass] ability is the one you are looking for. Shades have it. It also makes the unit able to stack with other units, though.
You can also use Permanent Invisibility with 0.00 second Hero and Unit Durations. Standard Abilities > Special > Units > Permanent Invisibility.
Ghost will make a unit passable through by other units. If it's Ghost (Visible), there is no invisibility but the passing through effect remains. Most invisibility abilities have their duration as their fade time, set this to zero and the unit won't reveal itself at all. Read here for more info on invisibilities.
I'm on a page about incorrect corrections, and spent the better part of like two hours trying to get someone to understand that -5^2 = -25, not 25, and then that post had comments get reposted because that group is self sustaining, and that person was in turn trying to explain what I just explained to them. And I'm taking that as a victory
I will be AFK for a couple of days you guys hold down the fort while I am gone. I will be checking in on my phone but that will severly limit me. Be back saturday!