The katana is unusable by the player and only seen being wielded by some Japanese soldiers during certain parts of the campaign mode. In the first mission, "Semper Fi", a Japanese soldier playing dead stabs Rooker with a katana. Another slashes Miller across the shoulder with a katana as he tries to escape with the other Marines. Also, in the last part of "Little Resistance", Sgt. Tom Sullivan gets killed by a Japanese Banzai Charger wielding a katana. Katanas are usually replaced with the Bayonet by the Japanese soldiers, however they can be seen on some of the soldiers' uniforms even if they are not used.
Examination of the game files reveals the menu and pickup icons for the katana, hinting it was originally going to appear as a usable weapon at some stage of development, at least until it got scrapped.
The Path of Sorrows can be obtained within the zombies map Revelations.
In order to obtain the weapon, the player must have progressed through the easter egg until they reach the point where the player can teleport to Samantha's room. Inside the spawn room is a glowing wisp on the headlight of a truck. Grab the wisp and bring it to the teleporter in Kino der Toten and teleport to The House. Inside the room is a custom Mystery Box with a candle on top of it. Place the wisp on the chest and it will sink into it. Grab the Kronorium on the bed and head over to the temple in Shangri-La, where the Path of Sorrows can be found on the wall for 500 points. It is pretty reliable as it can kill zombies with ease in the later rounds and it is a one hit kill until round 42.