Head to the White Knight's Castle, located in Falador, and climb up the stairs to the third floor. Speak with Sir Amik Varze.
Tell him that you seek a quest and he will inform you that they are receiving threats from the Black Knights. Normally that wouldn't be a worry, however they claim to have a secret weapon. He will offer you a mission to infiltrate the fortress in order to seek out and sabotage the weapon.
Accept and he will hand you a dossier to remind you of your mission (which will of course self destruct three seconds after reading it). Make sure you have a cabbage (that hasn't been picked from Draynor Village), and a bronze med helm and iron chainbody. If you don't have a cabbage, head north-east towards the Monastery and pick one. Now head into Ice Mountain and then travel north until you reach the Black Knights' Fortress.
Head to the west side of the fortress while wearing the armour, and enter through the sturdy door.
Straight in front of you will be a wall with an option to push. Push it and then climb up the ladder, and then climb the next ladder.
Now climb down the next ladder, go through the east door and climb up the ladder. Go down the next east ladder and then go through the door at the south and climb down once more.
Listen at the grill and you will hear the Witch talking about the secret weapon to a Black Knight. It will be mentioned that they are waiting for their final ingredient: a cabbage from Draynor Manor! The Witch will also say that a normal cabbage would entirely wreck the potion.
Head back to the entrance, and this time instead of pushing through the wall, enter through the door to the east. You will be stopped and told that the Black Knights are having a meeting, but choose the option that you don't care and head on through. You will be attacked, so either stay and fight or climb up the ladder. Head south-east and push through the wall.
You will now be in a room with a hole in the middle. Use your cabbage on it to sabotage the potion.
You have now completed your mission set by Sir Amik Varze, so head back to him in the castle to receive your reward.
Congratulations! Quest Complete!
Guide Made by: DaDa
Corrections submitted by: Deathslayer, DaDa, Yolk, Panda, Rebecca, Majinvegito3, Valerie