I dunno what causes this bug to happen... but it seems the 2 turns duration of invisibility is totally broken. My shadowblade char could stay hidden for more than 10 turns.
One could easily abuse this bug and use it with arrow spray to win all fights without even losing one single HP.
Turn invisible > Chill for 3 turns > Arrow spray in-your-face
Repeat above = Victory.
Guys, you gotta fix this.
