Temple Of Elemental Evil Assassin

[Jorgo]

How come Gremag wears chainmail even though he's an assassin? Does he get his assassin-y stuff penalized for it or what?

[Thrudd]

I think he's supposed to be "in disguise", the chainmail means people will think he's a fighter - because everyone knows thieves and assassins can't wear anything but leather or padded armor. It's not really clearly stated in the game what happens to a thief or assassin who puts on armor, the assumption is that they can't use any class abilities. In unearthed arcana, it lists penalties to thief skills for wearing different sorts of light armor, but not anything heavier than studded leather. Again, we'd presume that means no thieving is possible in heavier armor.
So you're houseruling whatever you think ought to happen - can the assassin backstab or assassinate with chainmail on? If you think they should be able to, sure. Or does he have to wait until night or when he's unobserved, slip out of the mail and then assassinate people? Penalty on stealthy skills while in armor, or no stealthy skills at all? Your call.