Acquire and Upgrade the KT-4

First, acquire the KT-4 by collecting three parts:

1. A vial, which spawns on a glowing zombie. Once killed, the vial will drop to be picked up.

2. A sample of Spider Venom, which can be obtained at Lab A. Approach the Panel adjacent to the cage and activate it - causing the cage to lower. Then, lure a spider into the cage, by standing near it. Return to the panel and activate it, upon which the spider will be killed and the venom will be produced.

3. A special plant, obtained in the underwater section past Mule Kick. Continue down through the watery cavern until reaching the bottom, where the plant can be found. (Note, it is recommended the player shoot the spore to the left of the plant in order to replenish their oxygen) 

With these three parts, locate the table in the room just before Mule Kick which has the KT-4 suspended on it, and add the three items, crafting the KT-4. If you lose the KT-4, it can be reacquired from the box.

To upgrade the KT-4, you need to acquire three more items and place them at a table adjacent to the original in the same room.

1. An upgraded vial, which can be obtained in a new area underneath Lab B.  First, every player in the game must complete their challenges at the Shrine of Nan Sapwe, to the far right from spawn. Once all challenges are complete, lightning will occasionally strike the shrine, and electrify the ground. A player with the shield should stand in this electricity with it out to electrify their shield. Now, go to the laboratory near Green Water, and electrify a panel near the cage. A player should step inside. In Solo, the cage will lower on its own. In Co-Op, another player must lower the cage. In the area where you are lowered, collect a vial inside a skeleton found there.

2. An upgraded sample of Spider Venom, which can be obtained by defeating the Spider mini-boss. Past the blue water outside Lab A, glowing webs can be found blocking a passageway. These must be shot with the KT-4, opening a slide down to the boss-arena. The Spider will shoot webs and spawn smaller spiders, and at some points its mouth will glow orange, at which point it should be shot there. It will jolt back after enough damage three times, before eventually dying. Once it dies, approach its mouth and collect one of its fangs (Note: there is no prompt for this). Exit the cave up ahead, passing a pool where you can obtain free Widows Wine.

3. From the room where  the KT-4 was initially built, use the fast travel pipe to Lab B. Midway through the travel, you will pass a spot with glowing rocks, and should interact at a precise moment to pick up rainbow water. You will know you have picked it up when the bucket icon appears full of water. Now return to the underwater cavern, and with the Skull of Nan Sapwe, mesmerize the wall to the right of where the plant was obtained. A hidden room will be revealed with a spot to plant a seed. Plant a seed, and water it with rainbow water for three consecutive rounds. You can then pick up the plant.

(The Rainbow Water can be obtained once this 115 rock is on screen)

Now you can craft the upgraded KT-4, the Masamune, which you will need for this quest by placing the three items at the table next to the original.