In Runescape, the Black Knight’s Fortress quest will be a medium quest, both in difficulty and in length of time that it takes to complete. You will need to have at least 12 quest points to get this quest. You’ll need a cabbage (that you didn’t get at Draynor Manor), an iron chainbody, and a bronze medium helm.
To start this quest you will need to talk to Sir Amik Varze in the White Knight’s Castle in Falador. You find him in the western tower on the third floor. You will be telling him that you need a quest and he will oblige you. Tell him you “laugh in the face of danger” and that you will do your best. He’ll give you a dossier afterwards.
Now, head to Black Knight’s Fortress north of Falador. If you didn’t have a cabbage before, you will need to pick one up out of the monastery that will be just due east of the Fortress. You will need to be wearing the helm and chainbody. Then enter through the sturdy door, push the wall, climb up the ladder you see twice and then climb the other ladder you’ll see that heads downward. There will be a door to the east, and you will go through it, and then see another ladder to climb up. After climbing that ladder up you will climb the next ladder down.
At that point you should be in a room with a chaos altar, use this to recharge your prayer points. After doing so go through the door and run to the ladder and head down it. You’ll have to be sneaky and use stealth to go along the passageway. Press on the vent grill so you can hear what they are saying about the invisible potion. They need a cabbage. (See why you were carrying a cabbage?)
You will have to go all the way back and then take the west door this time around. The guard will issue you a warning that you will be ambushed by black knights. Go through the door on the east and climb the ladder. Follow along the way and push on the strange wall you’ll see, use the cabbage on the hole.
You are done! Head back to Sir Amik Varze. You will complete the quest and he will give you 2500 gold and three quest points. A bit of hassle, but a good reward for it so it is well worth doing.