Starting Off
Talk to Sir Amik Varze, leader of the White Knights, on the third floor, west side, of the White Knight's castle. He will tell you that the Black Knights are making threats, and saying they have a new secret weapon. He asks you to infiltrate their Fortress, find, and destroy their secret weapon.
Black Knights Fortress
The fortress is location on the north side of Ice Mountain. To get into the Fortress, you need to use a side door, and have to be wearing a Bronze med helm and Iron chainbody.
Once inside, you'll need to go directly north and push on the hidden door (outlined in red on the nearby picture), and climb up the ladder, then climb up the other ladder. Climb down the nearby ladder. Climb up the next ladder, then down the next.
Enter the door near the chaos altar, and down the ladder through the door. Listen-at the grill to hear the Black Knights' plans. Head back up the stairs, and make your way to the ground floor where you first entered.
Cabbage!
Once down on the first floor, push through the hidden wall again, and enter the eastern room. A guard will warn you not to go in there, but go in anyway and head up the nearby ladder. Follow the path, towards the chaos altar (but obviously on the other side of the wall), and push through another fake wall into the room above the Witch. Use (be careful not to eat!) the cabbage with the hole in the room.
It will ruin the potion. Head back to Sir Amik Varze to claim your reward.
