How do you get the black hat with a white feather in runescape?

To obtain the black hat with a white feather in RuneScape, you need to complete the "Demon Slayer" quest. After finishing the quest, visit the Lumbridge Swamp and speak to the ghost of the former adventurer, who will give you the black hat. Equip it, and then you can attach a white feather from a chicken or another source to complete the look.