Katana +1 is an enchanted weapon. When its compared to a normal katana, it has +1 bonus to damage and THACO and the speed factor and the weight of the weapon are reduced with 1. In Baldur's Gate: Enhanced Edition, you can find one in Chapter 4. On the 3rd floor of the Cloakwood Mines, there is an Ogre Mage carrying a Katana +1.