Kusanagi-no-Tsurugi / KT-4
Built at the chemistry table holding the weapon frame in the bottom lab using:
Glowing Green Vial dropped from a zombified scientist (spawns in the Lab B area).
Spider Venom obtained from luring a Spider into the cage at Lab A.
Toxic Plant found underwater beyond the 115 chasms (Gas Mask slightly extends breathing time underwater and there is a spore at the bottom to return to the surface safely).
Upgrades to the Masamune using the second chemistry table in the same room using:
Liquid Divinium Vial found by meleeing the control panel to the cage in Lab B with an electrified Zombie Shield (requires all challenge Trials to be completed by all players).
Giant Spider Venom obtained from the fang of the Giant Spider after killing it.
Hidden Plant found by using the 'Mesmerize' function of the Skull of Nan Sapwe on the wall to the right of where you obtain the Toxic Plant, then planting a seed in the revealed planter and giving it "Rainbow Water" for three consecutive rounds until it grows into a second plant.
(And yes the HUD icons are identical to their predecessors except for Liquid Divinium which replaces the Glowing Green Vial HUD icon)
Skull of Nan Sapwe
At the Trials Altar four bloody humanoid skulls can be picked up (one per player); bring these to the correct of the four pedestals around the map and begin the "cleansing" ritual that will have you defend the pedestal from zombies (and Thrashers on the later two pedestals) attempting to attack it. Once all four skulls are cleansed and returned to the Trials Altar, a large tree will ascend revealing a stairwell to the Nan Sapwe Temple where you will have to fight Keepers after interacting with the Skull in order to obtain it.
First, the player must have completed all of their trials, they must have the electrified Zombie Shield and the Skull of Nan Sapwe. After doing all those things, the player must wait for a spider round to occur. The player must then use the Skull of Nan Sapwe to mesmerize the spiders to find the one that emits red mist. The player should then preferably kill all spiders except for the "red mist spider", to make the next steps easier. Once the player has identified the red mist spider, the player must then let it drink purple, green and blue Element 115-infected water by leading it to the water sources. After letting the spider drink the water, the player must catch it in a cage found at Lab A. Once the spider is caught, the player should pull the cage up to the next floor, then hit the control panel with the Electric Shield, and then lower the cage again, causing it to go underground this time. After that, the player must complete another spider round by killing all the spiders that spawn. This time, all the spiders will have red mist around them and they will be much more aggressive. Once the player kills all the spiders, the player must go back and raise the cage, the spider will now have been replaced by a cocoon. Then, extract the venom from the spider and whoever picked it up will get access to the Spider Bait, it will appear on the HUD and the player can now use it.