Katana

Katana are curved swords forged on the islands of Keikujo. Not all sword on Keikujo are katana, but these are the weapons of the samurai, and are therefore highly sought after.
Forging a katana requires heating steel to absurd temperatures and then folding it. This means that though they have razor sharp edges, they break easily. Some say each katana only take three lives before it breaks, though obviously this isn't set in stone.
Katana come in several shapes and sizes. The main katana is roughly the length of a longsword, while the wakizashi, which the samurai carry above their regular katana, is much shorter. There's also the oukatana and the tachi, which are both much larger than the average katana.
Predating the katana is the sraightsword. These weapons still appear around Keikujo, but they are usually old, and most often wielded by the Emishi.

5E D&D Rules

• Katana/Nagamaki = Longsword
• Wakizashi/Ninjato = Shortsword
• Tachi = Rapier
• Oukatana/Nodachi = Greatsword
• Tanto = Dagger (no throwing)

Feats

• Defensive Stance: You've mastered ways in which a single sword can be used for defense. Your constitution or dexterity increases by 1. While wielding a one-handed katana, you may hold the scabbard in your off hand for +2 to your AC. While wielding a two-handed katana, you may take a defensive stance as a bonus action, adding +1 to your AC.