Slayer Cape
Slayer Cape is the mastery cape of Slayer: only those who took that profession to level 100 wear it.
How to get it
Take Slayer to level 100 and talk to Grenda, veteran: they give you the quest Mastery: Slayer, and turning it in grants the cape. It is the only way to get it.
Who can use it
Any class can equip it.