Before heading to the Black Knights' Fortress, you'll need a cabbage, Medium Bronze Helmet, and an Iron Chain Mail Body. Cabbage can be picked up near Lumbridge or near the Monastery. Do not pick up cabbage from Draynor Manor as this cabbage does not work in the quest. You can buy a Medium Bronze Helmet from Peksa in Barbarian Village and an Iron Chain Mail Body from Horvik the Armourer in Varrock or Wayne in Falador. Once you have these three items, head to the Black Knights' Fortress, located on the north side of Ice Mountain and west of the Monastery.