A guild of cunning thieves sneaks their way into an opulent mansion, filled with the riches that only generations of power can acquire. The mansion's private guard patrol within, seeking out intruders and relentlessly hunting down those that their efforts reveal. Every shadow is a potential hiding spot, every noise might be a clue to the presence of thieves. Every second lost can mean the difference between victory and defeat.
When a thief is translucent, this effect is used so that team mates can still see them.
Against player guards will the thieves model fade as he resides into darkness (because human players can mess with contrasts and such to see him perfectly otherwise :P.