Solution to Rot Island

Start
At Port Sarim dock, speak to Crusty the Sailor there. Ask him why does it smell so bad that you want to throw up. He will tell you about Rot Island and its legends. Ask him if he can sail you there. He will, but you will collide on a rock and sink. Then, You and crusty will be swimming. Swim towards the shore with 30 Agility and you will be on Rot Island.

Rot Island
If you bring any food, they will turn into rotten food like Ghasts in Morytania but potions will not be affected. Then, you will find a Starving Explorer in the middle of Rot Island. Give him your water and he will explain about a level 176 Spirit Rot Dragon that made this island Rot Island. The dragon does not breathe fire but rot breath, and he explains about pressure armor that absorbs rot. He will give you a scroll and you will need it to make pressure stuff. Now, you have to choose your attack style.

Pressure armor
Go west of Rot Island and there should be 5 pressure slime clots. Each slime clot take 5 inventory spaces, so 1 clot mean 5 handfuls of pressure slime. Now, go into a hut with a worktable and sit on it, and use the chisel with the pressure slime. Then, go to a hut with a worktable and use it. You will only be able to make pressure armor. It costs 3 Pressure slime.

If you chose Melee
There are 3 steel bar respawns near the slime clots. You will need all 3, and then, go to a hut with an anvil, and use pressure slime on it. You will make a pressure sword. It costs 2 Pressure slime.

If you chose Magic
You will need pressure runes. The runecrafting altar can be found north-east of Rot Island. Instead of a talisman/tiara, you will need a pressure orb. You need 15 Presurre slimes to make a Pressure orb. Go to the Pressure obelisk at a hill south-west of Rot Island and you will craft a Pressure Orb. Now, go to the pressure altar and make your essence into pressure runes until you have enough. Next to the altar is a Pressure box. Put yourself into the box and you get the Pressure Spellbook. Currently you can cast a weak spell as weak as air strike that is called Pressure Strike. It requires 1 pressure rune.

If you chose Ranged
Chop the pressure trees that are all around Rot Island. Use a knife to cut them into pressure arrow shafts. Then, Go to another hut, and use pressure slime with the funny-looking anvil. 1 handful of pressure slime makes 50 pressure arrowheads. Now, collect pressure feathers from the Pressure Plant north of the hut with the Funny-looking anvil. Now, you have made Pressure Arrows. You do not need a bow.

The fight
If you're ready, go down the cave in the most northern place of Rot Island. You will see the dragon. Kill it, and Rot Island has been restored. It has been renamed "Pressure Island".

Rewards
Congratulations! Quest Complete!

Melee: Ranged: Magic:
 * Ability to use the pressure spellbook
 * Ability to use the pressure spellbook