Invisiblity spell

How can I make a unit always invisible besides when its attacking?

Ive seen it done many times, is it a passive skill or triggered?

The answer is within the far seer's wolves (level 3)

Hmm its stats are the same as ghost, but i guess ghost is coded differently.

ghost allows unit to move through other units (like windwalk) permenant invis doesnt

Is it possible to make invisible unit attacking while invis.?

lovely coincidence but it is possible if you add ghost and permanent invisibility at the same time

Perma Invis with a duration of 0. In the case of permanent invisibility "Duration" actually refers to the fadeout time of the unit with the ability.