My game version is 1.00 and in chapter 4 war zone after the fight with balor lord and wizards my 20/15/15/10 and 20/15 attack ratings decrease cumulatively. First I thought it was a bug and exported my character and started chapter 4 again but then after same part it became 4/-1/-6 ....Then I said OK and loaded my quicksave and saw it was changed.But it was OK when I saved it.
Does anyone know if this is a bug and I must download a patch or it's a spell(which I think it isn't).

Is your attack rating so low at your chracter sheet, or is it so low at your throwings at the bottom of your screen when you fight ?

I would start by downloading the latest patch, which is currently 1.30 (or buy SoU). There wre several issues that were resolved between 1.00 and now. Some of the problems involved the official campaign.