Permanent Invisibiliy Hero Ability

I need some help developing a permanent invisibility hero ability. The only thing I want to change with level is transition time, and heroes should be visibile when they attack. Any ideas?

... Changing Akama's permanent invisibility into a hero ability doesnt work, then?

Yeah that's what I was looking for I just couldn't pinpoint it. Sometimes I'm just not having a good day - thanks a lot