Drow Piwafwi Cloak offers the wearer a -6 bonus vs. Breath saving throws and +75% bonus for Hide in Shadows and Move Silently. However, this needed is to make progress in chapter 5 of Baldur's Gate II: Shadows of Amn and Baldur's Gate II: Enhanced Edition. The cloak needs to be given to Phaere as proof of Solaufein's demise by your hands, which means the cloak can only be used for a short period. Even then, the cloak would turn into Adamantine Dust just like any other drow item.