Black Nights Fortress

Official quest description: The black knights are up to no good. You are hired by the white knights to spy on them and destroy their secret weapon.
Start: Talk to Sir Amik Varze, located on the third (with the ground floor being the first) floor in the western tower of the White Knights’ Castle in Falador.
Difficulty: *****
Length: Medium
Members only: No
Requirements:
13 Quest Points
Capable of evading or killing with ease level 33 black knights.
Items needed:
A cabbage NOT from Draynor Manor (bring a spare in case you accidently eat the first one, the monastery is the closest location for this) You will need:
An iron chainbody (Horvik sells them for 210 coins if the Grand Exchange has none)
A bronze medium helmet (sold by Peksa if the Grand Exchange is out. A spawning point for one is found at the exit of Draynor Manor.)
Recommended Items
Some food [herring or better recommended] and strong armour (black or above) for lower level players
Monsters to kill: None
Rewards
3 Quest points
2500 coins
Spoiler warning: Plot and/or ending details follow.
Walkthrough
Obtain all items necessary for the quest before starting. They will fit in inventory and this will save time. Cabbage can be found at the monastery or purchased at the Grand Exchange.
Getting Started
Start the quest by talking to Sir Amik Varze on the 3rd floor of the White Knights' castle in Falador, the west side. Sir Amik asks you to 'check on' the Black Knights' Fortress (north side of Ice Mountain) and will give you a Dossier of information.
Members with a Combat Bracelet can teleport to the Monastery will get you almost right there. An Amulet of Glory teleport to Edgeville and running west along the Wilderness ditch is the next fastest way there.
Regardless, it isn't that far from Falador to the Black Knights' Fortress, which is located between Ice Mountain and the Monastery.
*** NOTE ***: If your character has a combat level lower than 66, there are a group of three black knights at the southern walkway to the fortress (toward the Dwarven Mine). This would be a good chance to see if you have the combat level necessary to take on the black knights inside of the fortress. These and all of the knights inside are aggressive, meaning they will attack you even though you haven't hit first. For those trying to avoid these knights, a separate entrance to the north of the fortress can be found in the wilderness, next to the Mind altar ruins. Don't let this scare you into thinking you must be above level 66 combat to complete this quest, as it is unnecessary to actually kill the black knights. Just be cautious if you are playing a character with low-level combat skills when approaching this fortress.

At the Fortress
Approaching the guards at the main doors of the fortress, an option to "Talk-To Fortress Guard" is available to find out what equipment is necessary for the disguise. In order to gain access to the fortress, you need the following items:
Bronze med helm
Iron chainbody
and for those wanting to complete the effect of looking just like one of guards you would want to get:
A black cape
Bronze sq shield
Any iron weapon
All completely optional, but adds flavour for infiltrating the fortress.
Once the bronze helm and iron chain-mail has been equipped, talking to the guards will give a response: "Hey! Get back on duty!". They think you are a guard and can enter the fortress.
Once inside, the disguise will no longer be necessary and you may switch to your better armour if you brought some (very strongly recommended!) Enter the Sturdy door on the south side of the fortress, and then push on the north wall you are facing to enter a secret area.
Climb up the ladder, then the ladder you find on the next floor. On this floor there are boxes blocking the way, so proceed down the eastern ladder. Enter the room to the east with a fenced off section and head up the ladder. Now head down the ladder just east of the one that was just climbed and you will be in a room with a Chaos altar. Proceed through the door west of the altar and then down the ladder, where you will be in a hallway behind the witch's room. At the end of this hallway 'listen at grill' to hear the devious plan of the black knights and the witch, and how to stop it.
[edit] Stopping the Evil Plan
Head back to the entrance of the fortress, enter the room to the east of the entrance (the one with the table in it). The guard will try to stop you by tell you that they are having a secret meeting and will kill anyone who enters. Respond that you don't care and you will enter the room. Low level players should be careful here, since there are five aggressive Black Knights in the room.
Quickly take the ladder up by passing the Black Knights. Walk to the end of this path and you will come across another wall that you can push. Enter this room and use, not eat, the cabbage on the hole. The cabbage will then fall straight into the potion, ruining five years worth of work by the witch.
Head back to Sir Amik Varze in Falador by any path you choose and talk with him. You will then be granted your reward.