@Glomoro : Not sure how things are in 5th ed since I've not played D&D since 3.5, but didn't they have the same stats as a Bastard Sword, but requiring Exotic Weapon Proficiency to use? I think it was 1D10 damage, unless I'm completely misremembering. They were definitely 1D10 back in AD&D (2nd ed). More than a 1D8 Longsword, but never really worth it because of the proficiency requirement and the rarity of the weapon type in the setting (i.e. unless you had them custom-made, everyone else would end up with better gear than you simply due to using more common weapon types).
NWN was 3.0. I never played NWN2, but that might have been 3.5.
I tried used Katanas in Baldur's Gate 2 (which was AD&D-based). Even tried a dual-wielding build for them. Problem was there were only two named/unique magical katanas in the entire game, a +2 and a +3... and it became really disadvantageous (without modding) by the time I got to the Throne of Bhaal content.